Cleanup ast_dtmf_stream()

* Made ast_dtmf_stream() wait after starting the silence generator rather
than before.

* Made ast_dtmf_stream() put the peer in autoservice for the whole time
things are being done to the chan.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373966 65c4cc65-6c06-0410-ace0-fbb531ad65f3