Add an option, specifying maximum analysis time for talk detection.

(closes issue #12149)
 Reported by: davevg
 Patches: 
       app_talkdetect.c.diff uploaded by davevg (license 209)
       (Plus a few additional cleanups by moi)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@123544 65c4cc65-6c06-0410-ace0-fbb531ad65f3