Fix ExtenSpy and simplify the channel search functions.

When ast_channel name was opaquified, the channel search functions did not
get converted correctly.  As a result ExtenSpy which uses a channel
iterator search by exten@context could never find anything.

* Updated the doxygen documentation for the search functions in channel.h.

Review: https://reviewboard.asterisk.org/r/1702/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@353647 65c4cc65-6c06-0410-ace0-fbb531ad65f3