refactoring tcp_output

[Kip Macy]

I've re-factored tcp_output into a number of smaller routines with the
intent of making it easier to work with. I would like feedback on how
to improve it. I've benchmarked the change on 10GigE with no
measurable performance degradation.

The diff can be found at:
http://www.fsmware.com/freebsd/tcp_output.diff

The actual file (with some additional changes that I'm not asking for
review of right now that are local to my branch for a) identifying the
egress queue and b) skipping the route lookup on *every* call to
ip_output and the corresponding 3 lock acquisitions) can be found at:
http://www.fsmware.com/freebsd/tcp_output.c

 -Kip