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/mtab.in

@@ -7,7 +7,7 @@ description="Update /etc/mtab to match what the kernel knows about"
 depend()
 {
 	need root
-	keyword -prefix
+	keyword -prefix -systemd-nspawn
 }
 
 start()