code style, formatting
	use enum/symbolic constants for return codes
	efficiency improvements
	(zaptel) only disable DTMF detection once per bridge, not every frame
	(zaptel) ensure VPM DTMF detector is turned off during native bridge


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@6312 65c4cc65-6c06-0410-ace0-fbb531ad65f3