udp: Log transmission and reception errors, at least some of the time

We keep a bitmask for each combination of
  - receive/send
  - address family
  - success/failure
and log a message the first time each one occurs.

We also provide a new utility function `af_name()' which is going to
be used by polypath's logging.

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