Improve compilation of SCTP code without INET support.
Some bugs where fixed while doing this:
- ASCONF-ACK messages might use wrong port number when using IPv6.
- Checking for additional addresses takes the correct address into account and also does not do more comparisons than necessary.
This patch is based on one received from bz@ who was
sponsored by The FreeBSD Foundation and iXsystems.
MFC after: 1 week