tcp: add some debug output

Also log, when dropping text or FIN after having received a FIN.
This is the intended behavior described in RFC 9293.
A follow-up patch will enforce this behavior for the base stack
and the RACK stack.
Reviewed by: rscheff
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44669