Enhance AGI with several fixes:

 - Makes the structures handling external AGI commands a bit more thread-safe
 - Makes AGI transparently work with both live and hungup channels
 - DeadAGI is hence no longer necessary and is deprecated
 - CLI bug fixes
 - Commands will refuse to run if the channel is dead and the command is nonsensical
   for dead channels.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76707 65c4cc65-6c06-0410-ace0-fbb531ad65f3