Add Duration header for PlayDTMF AMI Action

This patch adds an optional header to the PlayDTMF AMI action, Duration.
It allows the duration of the DTMF digit to be played on the channel to be
specified in milliseconds.

(closes issue ASTERISK-18172)
Reported by: Renato dos Santos

patches:
  send-dtmf.patch uploaded by Renato dos Santos (license #6267)

Modified slightly for this commit for Asterisk 12.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373979 65c4cc65-6c06-0410-ace0-fbb531ad65f3