Disable service scripts for systemd-nspawn

This adds the -systemd-nspawn keyword to service scripts which are not
intended to run in systemd-nspawn containers.

This fixes #52.

    X-Gentoo-Bug: 548058
    X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=548058

init.d/netmount.in

@@ -9,7 +9,7 @@ depend()
 	config /etc/fstab
 	use afc-client amd nfsclient autofs openvpn
 	use dns
-	keyword -jail -prefix -vserver -lxc
+	keyword -jail -prefix -systemd-nspawn -vserver -lxc
 }
 
 start()