netfilter: nft_meta: support for time matching

This patch introduces meta matches in the kernel for time (a UNIX timestamp),
day (a day of week, represented as an integer between 0-6), and
hour (an hour in the current day, or: number of seconds since midnight).

All values are taken as unsigned 64-bit integers.

The 'time' keyword is internally converted to nanoseconds by nft in
userspace, and hence the timestamp is taken in nanoseconds as well.

Signed-off-by: Ander Juaristi <a@juaristi.eus>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>