pydevd: explicit host/port for subprocesses to connect to

[int19h]

When pydevd detects a child process spawn and detours it, the child process currently connects to the same socket to which the parent process was connected.

To support #1706 in enable_attach() scenarios, the child process needs to be able to connect to a host/port that is explicitly specified, and not assumed to be the same.

The parent process will receive this information via a new property in the "launch" or "attach" request, e.g.:

"subProcesses": {
  "clientHost": "127.0.0.1",
  "clientPort": 7944,
}

(We'll use this property to aggregate all information that needs to be communicated to the parent about how it should handle the children, to avoid proliferation of top-level properties in debug config.)

It should propagate this information to the child process by whatever means are appropriate for the API used to spawn it - that part is considered pydevd implementation detail.

[fabioz]

Note: on hold (waiting for e-mail feedback).