Prevent unreal channels from optimizing during DTMF emulation

This prevents unreal channel optimization during the prequalification
phase when either channel is involved in DTMF emulation. This prevents
a situation where an emulated digit would be missed because the
emulation was never completed.

Review: https://reviewboard.asterisk.org/r/2747/
(closes issue ASTERISK-22214)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396385 65c4cc65-6c06-0410-ace0-fbb531ad65f3