Fix various mp bugs.

Fix various mp bugs.

In multiplaymgr.cxx:
The length of a string property that is too big was written out in the message
even though the string was truncated. Also, it was possible to overrun the
message buffer.

In incoming  messages, null-terminate the callsign to prevent any funny
business. Don't believe invalid string lengths. Turn the warning about unknown
properties from a warning into info; there are too many buggy clients out there,
not to mention people who add their own MP properties (hi vivian :)