Resolve an overlap in the ast_audiohook_flags values.

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@353598 65c4cc65-6c06-0410-ace0-fbb531ad65f3