comm: Break out some common udp parts

Provide a section in comm-common.h which allows other comms to share
some of the work done in udp.c.  Specifically, we provide the new
concepts of `udpsocks' and `udpcommon'.

The port configuration parameter is lifted into the udp struct.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>