tcp: Don't "negotiate" MSS.

_NO_ OSes actually "negotiate" MSS.

RFC 879:
"... This Maximum Segment Size (MSS) announcement (often mistakenly
called a negotiation) ..."

This negotiation behaviour was introduced 11 years ago by r159955
without any explaination about why FreeBSD had to "negotiate" MSS:

    In syncache_respond() do not reply with a MSS that is larger than what
    the peer announced to us but make it at least tcp_minmss in size.

    Sponsored by:   TCP/IP Optimization Fundraise 2005

The tcp_minmss behaviour is still kept.

Syncookie fix was prodded by tuexen, who also helped to test this
patch w/ packetdrill.

Reviewed by:    tuexen, karels, bz (previous version)
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D12430