upstream: allow certificate validity intervals, sshsig verification

upstream: allow certificate validity intervals, sshsig verification

times and authorized_keys expiry-time options to accept dates in the UTC time
zone in addition to the default of interpreting them in the system time zone.
YYYYMMDD and YYMMDDHHMM[SS] dates/times will be interpreted as UTC if
suffixed with a 'Z' character.

Also allow certificate validity intervals to be specified in raw
seconds-since-epoch as hex value, e.g. -V 0x1234:0x4567890. This
is intended for use by regress tests and other tools that call
ssh-keygen as part of a CA workflow.

bz3468 ok dtucker

OpenBSD-Commit-ID: 454db1cdffa9fa346aea5211223a2ce0588dfe13