Merge some downstream work.

[vodik]

Hi. A couple of my coworkers and I have been hacking on sipsak to add some features we need for our purposes. I've recently got it all moved up onto github and would like to spend the time to try and get these features merged upstream rather than maintain a separate fork. You can find out work here: https://github.com/sangoma/sipsak.

Code changes haven't been fully tested as at the moment we mainly use sipsak shoot support, but I'd like to round it out if there's interest:

• Initial IPv6 support (done to be as AF-independent as possible - lacking srv and raw socket support)
• Allow to set a specific a source address
• --retrans-limit to limit re-transmissions

[nils-ohlmeier]

Sorry I have not been actively working on sipsak for a long time. But since I recently started working on it again I would be happy to accept PR's now.
Since it looks like you haven't worked on sipsak-ng for quite some time I could also cherry pick changes from your branch if that is easier for you.

[vodik]

No worries, I'm caught up with a internal project at the moment with work, once its finished, I'll have some time to contribute our changes back.