Fix SendDTMF crash and channel reference leak using channel name parameter.

Fix SendDTMF crash and channel reference leak using channel name parameter.

The SendDTMF channel name parameter has two issues.
1) Crashes if the channel name does not exist.
2) Leaks a channel reference if the channel is the current channel.
Problem introduced by ASTERISK-15956.

* Updated SendDTMF documentation.

* Renamed app to senddtmf_name and tweaked the type.
........

Merged revisions 373945 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/1.8.15@376501 65c4cc65-6c06-0410-ace0-fbb531ad65f3