Unification and simplification of outbound TCP options handling

In tcp_syncache and tcp_output we have two entirely separate code paths dealing
with outbound TCP options.  Each of them has a significant size and many quirks.

This patch is a proper rewrite of both of them and introduces a new generic
function tcp_addoptions() to tcp_output.c.  The new function is then used in
both the syncache and the normal tcp_output path instead of their previous
versions.  It it alignment agnostic and can be used to write directly into
mbufs with any alignment.

The patch is here:

  http://people.freebsd.org/~andre/tcp_options-20070302.diff

Any testing and reviews appreciated.

-- 
Andre