Rename net.inet.ip.check_interface to rfc1122_strong_es and document it.

This very questionable feature was enabled in FreeBSD for a very short
time. It was disabled very soon upon merging to RELENG_4 - 23d7f14119bf.
And in HEAD was also disabled pretty soon - 4bc37f9836fb1.

The tunable has very vague name. Check interface for what? Given that
it was never documented and almost never enabled, I think it is fine
to rename it together with documenting it.

Also, count packets dropped by this tunable as ips_badaddr, otherwise
they fall down to ips_cantforward counter, which is misleading, as
packet was not supposed to be forwarded, it was destined locally.

Reviewed by: donner, kp
Differential revision: https://reviews.freebsd.org/D32912