MFC r367530:
RFC 7323 specifies that:

• TCP segments without timestamps should be dropped when support for the timestamp option has been negotiated.
• TCP segments with timestamps should be processed normally if support for the timestamp option has not been negotiated.

This patch enforces the above.
Manually resolved merge conflicts.

MFC 367891:
Fix an issue I introuced in r367530: tcp_twcheck() can be called
with to == NULL for SYN segments. So don't assume tp != NULL.
Thanks to jhb@ for reporting and suggesting a fix.

MFC r367946:
Fix two occurences of a typo in a comment introduced in r367530.
Thanks to lstewart@ for reporting them.

PR: 250499
Reviewed by: gnn, rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D27148