Another round of UDPTL stack fixes/improvements:

1) Allow users of UDPTL stack to associate a character-string tag with a UDPTL
   session, so that log/error/debug messages generated by the UDPTL stack can
   be 'connected' to the endpoint that caused them to be generated.

2) Improve comments (and process) of calculating the far end's maximum IFP size
   when redundancy mode is in use for error correction.

3) When an IFP larger than the calculated 'far max IFP' size is presented for
   writing, truncate it rather than putting in the buffer and allowing the buffer
   to overflow; this will cause the ends to retrain to a lower bit rate that
   produces IFPs of an appropriate size if possible, and if not possible, the
   FAX transfer will fail completely. In these cases, it is due to the one endpoint
   supplying a T38FaxMaxDatagram value that is improperly calculated and is
   too low to be of use; we have configuration options available to override
   this behavior.

4) Eliminate use of T38FaxMaxDatagram value in udptl.conf; it is no longer
   needed.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@231692 65c4cc65-6c06-0410-ace0-fbb531ad65f3