[doc/scripts/cert-staple.sh] *BSD date portability
[doc/scripts/cert-staple.sh] *BSD date portability

adjust date calculation for ocsp_expire for `date` command arguments
using different flags on FreeBSD, OpenBSD, and DragonFlyBSD

lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4