meta: don't use non-POSIX formats in strptime()

meta: don't use non-POSIX formats in strptime()

The current strptime() invocations in meta.c use the `%F` format which
is not specified by POSIX and thus unimplemented by some libc flavors
such as musl libc.

Replace all occurrences of `%F` with an equivalent `%Y-%m-%d` format
in order to be able to properly parse user supplied dates in such
environments.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>