Add better debugging output.

Add better debugging output.

All messages from UTCP itself are now of the form:

timestamp srcport:dstport foo 123 bar 456 ...

The UNIX timestamp is wall clock time, to allow data from multiple
computers that are synchronised via NTP to be correlated.
Only part of the payload is now dumped by default to make the logs more
easily readable.

The test program now also adds the same format timestamp to its debug
messages.