sshd: address capsicum issues

sshd: address capsicum issues

* Add a wrapper to proxy login_getpwclass(3) as it is not allowed in
  capability mode.
* Cache timezone data via caph_cache_tzdata() as we cannot access the
  timezone file.
* Reverse resolve hostname before entering capability mode.

PR:		231172
Submitted by:	naito.yuichiro@gmail.com
Reviewed by:	cem, des
Approved by:	re (rgrimes)
MFC after:	3 weeks
Differential Revision:	https://reviews.freebsd.org/D17128