Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add ng_tcpmss node, which alters MSS options of TCP packets. Useful to
hack MSS of packets outgoing via interface with small MTU, to workaround
path MTU discovery problems.

Written by Alexey Popov, with some cleanups from me. There are also plans
to improve mpd port, so that it uses this node, instead of doing MSS
hacking in userland, when 'enable tcpmssfix' option is on.

Submitted by: Alexey Popov <lollypop@flexuser.ru>