Clean transmit_* for start/stop media transmission

Small patch changing skinny_set_rtp_peer to use transmit_stopmediatransmission and to use new transmit_startmediatransmission.
Basic testing on 30VIP's by wedhorn
Basic testing on 7960 by me

(closes issue #16956)
Reported by: wedhorn
Patches:
      skinny-clean05b.diff uploaded by wedhorn (license 30)
Tested by: wedhorn,mvanbaak


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@251222 65c4cc65-6c06-0410-ace0-fbb531ad65f3