The handling of RST segments in the SYN-RCVD state exists in the
code paths. Both are not consistent and the one on the syn cache code
does not conform to the relevant specifications (Page 69 of RFC 793
and Section 4.2 of RFC 5961).

This patch fixes this:

• The sequence numbers checks are fixed as specified on page Page 69 RFC 793.
• The sysctl variable net.inet.tcp.insecure_rst is now honoured and the behaviour as specified in Section 4.2 of RFC 5961.

Approved by: re (gjb@)
Reviewed by: bz@, glebius@, rrs@,
Differential Revision: https://reviews.freebsd.org/D17595
Sponsored by: Netflix, Inc.