pydevd: reconnect listener

[int19h]

It should be possible to start pydevd in the mode whereby it opens a listener socket for incoming DAP connections, regardless of the direction of the first connection that it establishes to the client (i.e. even if it's pydevd -> client, and not client -> pydevd). That listener socket should refuse connections for as long as there is one already active, but should accept new connections otherwise.

To enable this mode, we'll use a custom property in the "initialize" request:

{
  ...
  "command": "initialize",
  "arguments": {
    ...
    "pydevd": {
       "serverHost": ...,   // default "127.0.0.1"
       "serverPort": ...,   // can be 0 to pick an available ephemeral port
    }
  }
}

(we'll use the same top-level property for any other custom pydevd-related initialization bits in the future)

The listener socket is enabled if "serverPort" is specified - to pick some available free port, the other side must pass 0.

pydevd must open the listener socket and invoke listen() on it before sending the "initialize" response. The response should contain information about the host and the port on which the socket is actually listening, as returned by getsockname() (which might be different from inputs in case "0.0.0.0" or :0 are used):

{
  ...
  "command": "initialize",
  "success": true,
  "body": {
    ...
    "pydevd": {
       "processId": ...,    // pid of the child process
       "serverHost": ...,   // actual host
       "serverPort": ...,   // actual port
    }
  }
}

[int19h]

It would also be nice to always include "processId" as a property in "initialize" response, even in non-multiproc scenarios. This will make it a little bit easier to handle this all uniformly in the adapter.

[fabioz]

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