Add 'zap set dnd' CLI command, and ensure that the AMI DNDState event always gets generated.

(closes issue #11212)
Reported by: tzafrir
Patches:
      zap_dnd.diff uploaded by tzafrir (modified by me) (license 46)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@98488 65c4cc65-6c06-0410-ace0-fbb531ad65f3