MFC r304736:
When aborting an association, send the ABORT before notifying the upper
layer. For the kernel this doesn't matter, for the userland stack, it does.
While there, silence a clang warning when compiling it in userland.

MFC r304837:
Fix a bug, where no SACK is sent when receiving a FORWARD-TSN or
I-FORWARD-TSN chunk before any DATA or I-DATA chunk.

Thanks to Julian Cordes for finding this problem and prividing
packetdrill scripts to reporduce the issue.