MFC r338138:

Enabling the IPPROTO_IPV6 level socket option IPV6_USE_MIN_MTU on a TCP
socket resulted in sending fragmented IPV6 packets.

This is fixes by reducing the MSS to the appropriate value. In addtion,
if the socket option is set before the handshake happens, announce this
MSS to the peer. This is not stricly required, but done since TCP
is conservative.

PR: 173444
Reviewed by: bz@, rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16796