We used the approxidate() parser for "--expire=<timestamp>" options
of various commands, but it is better to treat --expire=all and
--expire=now a bit more specially than using the current timestamp.
Update "git gc" and "git reflog" with a new parsing function for
expiry dates.

* jc/prune-all:
  prune: introduce OPT_EXPIRY_DATE() and use it
  api-parse-options.txt: document "no-" for non-boolean options
  git-gc.txt, git-reflog.txt: document new expiry options
  date.c: add parse_expiry_date()