Split tcp_mss() in tcp_mss() and tcp_mss_update() where the former

Split tcp_mss() in tcp_mss() and tcp_mss_update() where the former
calls the latter.

Merge tcp_mss_update() with code from tcp_mtudisc() basically
doing the same thing.

This gives us one central place where we calcuate and check mss values
to update t_maxopd (maximum mss + options length) instead of two slightly
different but almost equal implementations to maintain.

PR:		kern/118455
Reviewed by:	silby (back in March)
MFC after:	2 months