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.
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Merged revisions 373945 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373946 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373954 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373965 65c4cc65-6c06-0410-ace0-fbb531ad65f3