Add option to followme to delay answer until ready to bridge call.

Followme answers an incoming call if it hasn't already been answered and starts
MOH. Some poorly designed autodialers see the answer and start playing their
message to the hold music. The 'N' option has been added to indicate ringing and
not answer until the call is accepted.

(closes issue #18479)
Reported by: ianc
Patches: 
      trunk_followme.diff uploaded by ianc (license 998)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@304384 65c4cc65-6c06-0410-ace0-fbb531ad65f3