Allow SendDTMF to play digits to a specified channel.

Patch supplied by reporter was modified to use autoservice and
prevent a potential channel ref leak but is otherwise as the
reporter uploaded it.

(closes issue #17182)
Reported by: rcasas
Patches:
      app_senddtmf.c.patch_trunk uploaded by rcasas (license 641)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@265453 65c4cc65-6c06-0410-ace0-fbb531ad65f3