Draft: sip: Enable IPv6 for signalling and media handling
This commit removes a lot of unnecessary code. For the media pipeline we decide if our listening socket should use IPv6 by looking at the offered SDP message.
Closes #278
Draft because !402 (merged) should be in first
However, testing was successful in my network (see !383 (closed) - The old is gone because I forked again after having cleaned up hundreds of stale branches in the main repository)