Provide counter_warning(9) KPI which allows to issue limited number of
warnings for some kernel events, mostly intended for the use of
obsoleted or otherwise undersired interfaces.

This is an abstracted and race-expelled code from compat pty driver.

Requested and reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D7270