bridge_multiplexed: Rename variables so they are not the same as the struct name.

* Rename multiplexed_thread variables to muxed_thread.  It is shorter and
my editer tagging works much better.  Struct names and variable names have
different purposes and therefore should have different names.

* Renamed the multiplexed_threads container to muxed_threads for
consistency.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380142 65c4cc65-6c06-0410-ace0-fbb531ad65f3