Add ability to adapt the IAX trunk packets to the MTU size, to avoid bad audio

when the number of channels fill the MTU on a given link.

In the future, this needs to be configurable per peer with trunking enabled.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46195 65c4cc65-6c06-0410-ace0-fbb531ad65f3