MFC r256556

Remove a buggy comparision when setting manually the path MTU.
After fixing, the comparision would have become redundant.
Thanks to Andrew Galante for reporting the issue.
MFC r257272:

Fix compilation if SCTP_DONT_DO_PRIVADDR_SCOPE is defined.
The issue was reported by Andrew Galante.
MFC r257274:

Fis the value of *optlen when calling getsockopt() for
SCTP_REMOTE_UDP_ENCAPS_PORT.
This issue was reported by Andrew Galante.
MFC r257359:

Terminate a debug output with a \n.
MFC r257555:

Changes from upstream to improve compilation when INET or INET6
or none of them is defined.
MFC r257574:

Unlock the lock before destroying it.
This issue was reported by Andrew Galante.