Use single global c line when creating SDP

Ignore-this: 13f6a839e12e15d1632553c15ba1d96

PJSIP will internally create a c line, but if a VPN is used it may
contain the wrong address and will confuse endpoints.

darcs-hash:20130819112833-56b4d-0180595314a1918effab8ceba13cbc99ac3cd0c4