translate.c: Only select audio codecs to determine the best translation choice.

Given a source capability of h264 and ulaw, a destination capability of
h264 and g722 then ast_translator_best_choice() would pick h264 as the
best choice even though h264 is a video codec and Asterisk only supports
translation of audio codecs.  When the audio starts flowing, there are
warnings about a codec mismatch when the channel tries to write a frame to
the peer.

* Made ast_translator_best_choice() only select audio codecs.

* Restore a check in channel.c:set_format() lost after v1.8 to prevent
trying to set a non-audio codec.

This is an intermediate patch for a series of patches aimed at improving
translation path choices for ASTERISK-24841.

This patch is a complete enough fix for ASTERISK-21777 as the v11 version
of ast_translator_best_choice() does the same thing.  However, chan_sip.c
still somehow tries to call ast_codec_choose() which then calls
ast_best_codec() with a capability set that doesn't contain any audio
formats for the incoming call.  The remaining warning message seems to be
a benign transient.

ASTERISK-21777 #close
Reported by: Nick Ruggles

ASTERISK-24380 #close
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/4605/
........

Merged revisions 434614 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434615 65c4cc65-6c06-0410-ace0-fbb531ad65f3

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