This gets very close to avoiding any possible hangs to do with tty
locking and SIGINT handling minus a special case that will be detailed
below.

Summary of implementation changes:

- convert `_mk_pdb()` -> `with _open_pdb() as pdb:` which implicitly
  handles the `bdb.BdbQuit` case such that debugger teardown hooks are
  always called.
- rename the handler to `shield_sigint()` and handle a variety of new
  cases:
  * the root is in debug but hasn't been cancelled -> call
    `Actor.cancel_soon()`
  * the root is in debug but *has* been called (`Actor.cancel_soon()`
    already called) -> raise KBI
  * a child is in debug *and* has a task locking the debugger -> ignore
    SIGINT in child *and* the root actor.
- if the debugger instance is provided to the handler at acquire time,
  on SIGINT handling completion re-print the last pdb++ REPL output so
  that the user realizes they are still actively in debug.
- ignore the unlock case where a race condition of "no task" holding the
  lock causes the `RuntimeError` normally associated with the "wrong
  task" doing so (not sure if this is a `trio` bug?).
- change debug logs to runtime level.

Unhandled case(s):

- a child is maybe in debug mode but does not itself have any task using
  the debugger.
    * ToDo: we need a way to decide what to do with
      "intermediate" child actors who themselves either are not in
      `debug_mode=True` but have children who *are* such that a SIGINT
      won't cause cancellation of that child-as-parent-of-another-child
      **iff** any of their children are in in debug mode.

Using either of `@pdb.hideframe` or `__tracebackhide__` on stdlib
methods doesn't seem to work either.. This all seems to have something
to do with async generator usage I think ?

None of it worked (you still will see `.__exit__()` frames on debugger
entry - you'd think this would have been solved by now but, shrug) so
instead wrap the debugger entry-point in a `try:` and put the SIGINT
handler restoration inside `MultiActorPdb` teardown hooks.

This seems to restore the UX as it was prior but with also giving the
desired SIGINT override handler behaviour.

Finally! I think this may be the root issue we've been seeing in
production in a client project.

No idea yet why this is happening but the fault-causing sequence seems
to be:
- `.open_context()` in a child actor
- enter the debugger via `tractor.breakpoint()`
- continue from that entry via `c` command in REPL
- raise an error just after inside the context task's body

Looking at logging it appears as though the child thinks it has the tty
but no input is accepted on the REPL and a further `ctrl-c` results in
some teardown but also a further hang where both parent and child become
unresponsive..

There's a bug that's triggered in the stdlib without latest `pdb++`
installed; add a note for that.

Further inside `wait_for_parent_stdin_hijack()` don't `.started()` until
the interactor stream has been opened to avoid races when debugging this
`._debug.py` module (at the least) since we usually don't want the
spawning (parent) task to resume until we know for sure the tty lock has
been acquired. Also, drop the random checkpoint we had inside
`_breakpoint()`, not sure it was actually adding anything useful since
we're (mostly) carefully shielded throughout this func.

There's no point in sending a cancel message to the remote linked task
and especially no reason to block waiting on a result from that task if
the transport layer is detected to be disconnected. We expect that the
transport shouldn't go down at the layer of the message loop
(reconnection logic should be handled in the transport layer itself) so
if we detect the channel is not connected we don't bother requesting
cancels nor waiting on a final result message.

Why?

- if the connection goes down in error the caller side won't have a way
  to know "how long" it should block to wait for a cancel ack or result
  and causes a potential hang that may require an additional ctrl-c from
  the user especially if using the debugger or if the traceback is not
  seen on console.
- obviously there's no point in waiting for messages when there's no
  transport to deliver them XD

Further, add some more detailed cancel logging detailing the task and
actor ids.

The method now returns a `bool` which flags whether the transport died
to the caller and allows for reporting a disconnect in the
channel-transport handler task. This is something a user will normally
want to know about on the caller side especially after seeing
a traceback from the peer (if in tree) on console.

A hopefully significant fix here is to always avoid suppressing a SIGINT
when the root actor can not detect an active IPC connections (via
a connected channel) to the supposed debug lock holding actor. In that
case it is most likely that the actor has either terminated or has lost
its connection for debugger control and there is no way the root can
verify the lock is in use; thus we choose to allow KBI cancellation.

Drop the (by comment) `try`-`finally` block in
`_hijoack_stdin_for_child()` around the `_acquire_debug_lock()` call
since all that logic should now be handled internal to that locking
manager. Try to catch a weird error around the `.do_longlist()` method
call that seems to sometimes break on py3.10 and latest `pdbpp`.

See issues:
- pdbpp/pdbpp#480
- pdbpp/pdbpp#482

Ensure that even when `pdb` resumption methods are called during a crash
where `trio`'s runtime has already terminated (eg. `Event.set()` will
raise) we always revert our sigint handler to the original. Further
inside the handler if we hit a case where a child is in debug and
(thinks it) has the global pdb lock, if it has no IPC connection to
a parent, simply presume tty sync-coordination is now lost and cancel
the child immediately.

See pdbpp/pdbpp#496

When in an uncertain teardown state and in debug mode a context can be
popped from actor runtime before a child finished debugging (the case
when the parent is tearing down but the child hasn't closed/completed
its tty lock IPC exit phase) and the child sends the "stop" message to
unlock the debugger but it's ignored bc the parent has already dropped
the ctx. Instead we call `._debug.maybe_wait_for_deugger()` before these
context removals to avoid the root getting stuck thinking the lock was
never released.

Further, add special `Actor._cancel_task()` handling code inside
`_invoke()` which continues to execute the method despite the IPC
channel to the caller being broken and thus avoiding potential hangs due
to a target (child) actor task remaining alive.

The common logic to both remove our custom SIGINT handler as well
as signal the actor global event that pdb is complete. Call this
whenever we exit a post mortem call and thus any time some rpc task
get's debugged inside `._actor._invoke()`.

Further, we have to manually print the REPL prompt on 3.9 for some wack
reason, so stick a version guard in the sigint handler for that..

After many tries I just don't think it's worth it to make the tests work
since the repl UX in `pdbpp` is so unreliable in the latest release and
honestly we're trying to go 3.10+ ASAP.

Further,
- entirely drop the pattern matching inside the `do_ctlc()` for now.
- add a `subactor_error` parametrization that catches a case that
  previously caused a hang (when you use 'next' immediately after the
  first crash/debug lock (the fix was pushed just before this commit).