Resolve an overlap in the ast_audiohook_flags values.

AST_AUDIOHOOK_TRIGGER_WRITE and AST_AUDIOHOOK_WANTS_DTMF were overlapping which
may have caused unintended side effects.  This patch moves
AST_AUDIOHOOK_TRIGGER_WRITE, and updates AST_AUDIOHOOK_TRIGGER_MODE to reflect
the original intention.

This will affect existing modules that use these flags, so be sure to recompile
as necessary.

(closes issue ASTERISK-19246)
Reported by: feyfre
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Merged revisions 353598 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 353599 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@353600 65c4cc65-6c06-0410-ace0-fbb531ad65f3