func_channel: dtmf_features setting

Allows reading andsetting dtmf features via a channel function
CHANNEL(dtmf_features)

(closes issue ASTERISK-21876)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2648/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395215 65c4cc65-6c06-0410-ace0-fbb531ad65f3