Teach nitcorn to listen to an unbinden port

[privat]

Sometime, especially for tests, we want that a server listen to any available port.

The C socket API features this possibility:

• either bind to the port 0
• or just do not bind() (directly call listen())

To get the port (to communicate it to the client), use getsockname() after the listen()

Related to #2084 and #2031

[privat]

@jpcaissy Can you give us your insight about this one since you know the codebase?