net-firewall/nftables: update systemd file target.

net-firewall/nftables: update systemd file target.

Change the systemd unit files to use

    Before=network-pre.target
    Wants=network-pre.target

as this better aligns with systemd documentation for this type of
service.  Special thanks to Shaun Bouckaert for triggering this review.
He originally asked (via email) about how the connman systemd unit file
handled its dependencies which prompted me to check this package as
well.  As for connman, v1.33-r1 seems to be aligned with systemd
documentation.

Package-Manager: portage-2.3.2
Closes: https://github.com/gentoo/gentoo/pull/2649

Signed-off-by: Patrice Clement <monsieurp@gentoo.org>