Optionally treat EOF on stdin just like SIGTERM

[bassosimone]

On Unix, Node.js allows us to gracefully kill a process. On Windows
this is more compex. You certainly cannot rely on the default kill()
function, which calls TerminateProcess.

There is a bunch of C/C++ extensions that in principle allow you to
attempt to gracefully shutdown a Windows process.

But, hey, here's a reality check. Node.js controls our stdin. Node.js
does IPC easy. Controlling uv_spawn flags and using the right not well maintained
C/C++ Node.js extension to kill a process is fragile.

So, treat EOF and any other error on stdin as equivalent to SIGTERM.

However, systemd.

The sane thing to do with systemd is StandardInput=null. With such
configuration, stdin immediately returns EOF.

Then, introduce the OONI_STDIN_EOF_IMPLIES_SIGTERM environment
variable. When it is true, this behaviour is enabled, e.g.:

export OONI_STDIN_EOF_IMPLIES_SIGTERM=true  # behaviour enabled
ooniprobe run

I want the default to be disabled because:

1. in the future we may find a better way to solve this problem and I
   don't want the default behaviour to change in such case

2. we know we need this knob for ooniprobe-desktop, and we will not
   fail to provide it, so it won't suprise/damage us

3. a person trying to write a systemd unit for ooniprobe would be very
   surprised to find out they need to disable this behaviour, if it was
   enabled by default by this PR

Hence, I believe this design is consistent with designing for the
future and for trying to minimize surprises.

Also, why an environment variable and not a command line flag? Because:

1. we don't want such hypothetical flag to be available where it does not
   make sense, e.g., for all subcommands but run

2. we don't want the ooni/probe-desktop app to write conditional
   code because it needs to check the command we're using and then decide
   whether to add such hypothetical flag

Also, why not enabling this only on Windows? Because again we don't
want the ooni/probe-desktop app to write conditional code.

To summarize: we want ooni/probe-desktop app to see the same behaviour
everywhere and we want others to be the least surprised.

Related to ooni/probe#1005

[hellais]

Great work!

Thanks for providing a comprehensive explanation of how it works too.

I left just a very minor nitpick on how to improve the comment, but this looks good to be merged!