func_channel.c: Add missing locking to some CHANNEL() requests.

* The CHANNEL() audionativeformat, videonativeformat, audioreadformat, and
audiowriteformat now need locking since the media format rework when
accessing the channel's format pointers.

* Increased the buffer size for CHANNEL() audionativeformat and
videonativeformat output strings since the allow=all can be a lengthy
list.

* Tweaked the CHANNEL() XML documentation for secure_bridge_signaling,
secure_bridge_media, and state.

* Ensured the output buffer is initialized for secure_bridge_signaling and
secure_bridge_media.

* Made use the locked_copy_string() macro instead of inlining it for trace
and checkhangup.
........

Merged revisions 422700 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422701 65c4cc65-6c06-0410-ace0-fbb531ad65f3