Added support for reading the TOUCH_MONITOR_PREFIX channel variable.

It allows you to configure a prefix for auto-monitor recordings.

(closes issue #6353)
Reported by: ivanfm
Patches: 
      asterisk_automon_v4.patch uploaded by ivanfm (original patch)
	   - updated patch:
         6353-touch_monitor_prefix.diff uploaded by qwell (license 4)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@85682 65c4cc65-6c06-0410-ace0-fbb531ad65f3