Added music_on_hold paramater for conference. Handle hold/resume in conference correctly. When moderator is onhold, music is player. Set conf_music_on_hold_class to empty string, if music on hold is not wanted.

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4183 43c1dc8c-776e-4df2-8ef1-6e829299be21