tcp: introduce common support code for TCP socket communication

tcp: introduce common support code for TCP socket communication

It appears that so far only SCPI and the Beagleboard have supported
communication across TCP sockets. And both have locally open coded
the corresponding logic.

Introduce common support for TCP communication. This implementation
provides simple new/free, connect/disconnect, read/write (including
non-blocking reads), source add/remove calls, and keeps the API under
the sr_tcp_ prefix.

This commit introduces common code, but does not adjust callers.