strptime: support everything that strftime supports.

Our strptime was missing `%F`, `%G`, `%g`, `%P`, `%u`, `%V`, and
`%v`. Most of these are already supported upstream (and I've just pulled
their current implementation), but some aren't. We're horribly out of
sync anyway, so I'll upstream the missing pieces and then try to get us
back in sync later.

Test: new tests, but originally found by toybox trying to use %F
Change-Id: Ib1a10801a7a3b9c9189440c3b300109bde535fd9