Cleanup transmit_* functions, part 1

Break transmit_tone into transmit_start_tone and transmit_stop_tone as per the skinny protocol. 

(closes issue #16874)
Reported by: wedhorn
Patches:
      skinny-clean01.diff uploaded by wedhorn (license 30)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@248184 65c4cc65-6c06-0410-ace0-fbb531ad65f3