Add 'auto' signalling mode for Zaptel channels.

(closes issue #11690)
Reported by: tzafrir
Patches:
      signaling_to_signalling.diff uploaded by tzafrir (license 46)
      signalling_cleanup.diff uploaded by tzafrir (license 46)
      zap_auto_default.diff uploaded by tzafrir (license 46)
      zap_no_default_sig.diff uploaded by tzafrir (license 46)
      zap_signal_auto.diff uploaded by tzafrir (license 46)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@98436 65c4cc65-6c06-0410-ace0-fbb531ad65f3