Add extra functionality to AGI command WAIT FOR DIGIT.

Add the ability to play a sound file, listen for more than just one digit,
specify
escape characters. Backwards compatible (to work with only timeout specified).

(closes issue #15531)
Reported by: diLLec
Patches:
      asterisk-res_agi-203638-patched.patch uploaded by diLLec (license 839)
Tested by: diLLec, espiceland



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@295554 65c4cc65-6c06-0410-ace0-fbb531ad65f3