Implement nftables.service for static firewall configurations (bsc#1237277)

It seems users are missing this service which is not part of the
upstream project but present in most other Linux distributions. It
allows to setup simple static nftables based firewalls via configuration
files.

nftables.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=nftables static rule set
+Documentation=file:/usr/share/doc/packages/nftables/README.SUSE
+Wants=network-pre.target
+Before=network-pre.target shutdown.target
+Conflicts=shutdown.target
+DefaultDependencies=no
+AssertPathExists=/etc/nftables/rules/main.nft
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+StandardInput=null
+ProtectSystem=full
+ProtectHome=true
+AssertPathExists=/etc/nftables/rules/main.nft
+ExecStart=/usr/sbin/nft -f /etc/nftables/rules/main.nft
+ExecReload=/usr/sbin/nft -f /etc/nftables/rules/main.nft
+ExecStop=/usr/sbin/nft flush ruleset
+
+[Install]
+WantedBy=sysinit.target