prune: introduce OPT_EXPIRY_DATE() and use it

prune: introduce OPT_EXPIRY_DATE() and use it

Earlier we added support for --expire=all (or --expire=now) that
considers all crufts, regardless of their age, as eligible for
garbage collection by turning command argument parsers that use
approxidate() to use parse_expiry_date(), but "git prune" used a
built-in parse-options facility OPT_DATE() and did not benefit from
the new function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>