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