media: MSOC 2007 compatibility refactor II
New handling of OC2007 compatibility:
First create only 2007 R2 compatible call in backend and send
multipart/alternative INVITE with empty 2007fallback part (only media
description without codecs or candidates).
When OC 2007 client parses the message, it responds with 488
Not Acceptable Here. If this happens, we retry the call once again,
this time using ICEv6, that is compatible with OC2007's older release.
On another error response we give up trying with an error message.
OC2007 R2 call establishement is left unchanged.