comm: Formalise interface to udp sockets

Have the poll registration done by the udpcommon/udpsocks code, rather
than by udp.c.  This means we can abolish the two wrapper functions,
but we do need an extra pointer in a udpsocks to find the udpcommon.

No overall functional change.

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