Add socket listener API

[wez]

Provide a convenient but still lowish level API for defining socket based servers and listening for connections.

Need to be able to specify:

• Local sockaddr to bind to
• listen(2) backlog
• Common socket options (eg: SO_REUSEADDR)
• Callback function to receive the newly connected client socket and phenom sockaddr

It is desirable to enumerate all the active listeners so that they can be shutdown safely in response to the software closing down, or an operator performing an explicit shutdown of a specific listener.