Add a new dialplan function, DIALPLAN_EXISTS(), which allows you to check for

the existence of a dialplan target.

(closes issue #11579)
Reported by: irroot
Patches: 
      func_dialplan2.c uploaded by irroot (license 52)
	  -- Additional changes by me.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@94799 65c4cc65-6c06-0410-ace0-fbb531ad65f3