Re-add 'o' option to MeetMe, reverting rev 62297.

Enabling this option by default proved to be a bad idea, as the talker detection
is not very reliable.  So, make it optional again, and off by default.

(issue #13801)
Reported by: justdave


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@177101 65c4cc65-6c06-0410-ace0-fbb531ad65f3