tools: add a systemd unit for static rulesets

tools: add a systemd unit for static rulesets

There is a customer request (bugreport) for wanting to trivially load a ruleset
from a well-known location on boot, forwarded to me by M. Gerstner. A systemd
service unit is hereby added to provide that functionality. This is based on
various distributions attempting to do same, for example,

https://src.fedoraproject.org/rpms/nftables/tree/rawhide
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/nftables/nftables.initd
https://gitlab.archlinux.org/archlinux/packaging/packages/nftables
Acked-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>