Updates to udp source/sink (select(), wait, cleanup)

Updates to udp source/sink (select(), wait, cleanup)

Use select() to avoid blocking on recv() in gr_udp_source (only known
way to avoid blocking on Cygwin).

Add wait argument to gr_udp_source to allow waiting for connection
or accepting lack of connection as EOF; add --no-wait option to
dial_tone_sink.py.

Remove system dependencies from .h files; remove unused data members
and (useless?) public open and close functions.