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