chan_iax2 has an extremely large function, socket_process(), to handle incoming

chan_iax2 has an extremely large function, socket_process(), to handle incoming
frames.  The function, before this commit, was roughly 1400 lines long.  So, I
am working on breaking this up into functions so that the code is easier to
follow and debug.  Also, I will be committing these changes in chunks as I do
them to ease tracking down any potentially introduced problems.

Break out roughly 150 lines from socket_process() and introduce a new function,
socket_process_meta() which handles the parsing of an incoming meta frame.
Also, restructure some of this code a bit to reduce the deep nesting that was
in this code.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48360 65c4cc65-6c06-0410-ace0-fbb531ad65f3