tcp: Fix an issue with the PUSH bit as well as fill in the missing mtu change for fsb's

The push bit itself was also not actually being properly moved to
the right edge. The FIN bit was incorrectly on the left edge. We
fix these two issues as well as plumb in the mtu_change for
alternate stacks.

Reviewed by: mtuexen
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30413

(cherry picked from commit 631449d5d03506295eaa6947c1b0e8a168a2f6b7)