Retain only mutually supported TCP options after simultaneous SYN

When receiving a parallel SYN in SYN-SENT state, remove all the
options only we supported locally before sending the SYN,ACK.

This addresses a consistency issue on parallel opens.

Also, on such a parallel open, the stack could be coaxed into
running with timestamps enabled, even if administratively disabled.

Reviewed by: tuexen (mentor)
Approved by: tuexen (mentor)
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D23371