systemd: Move D-Bus conf file to share/dbus-1/system.d

Since D-Bus 1.9.18 configuration files installed by third-party should
go in share/dbus-1/system.d. The old location is for sysadmin overrides.

pkgs/os-specific/linux/systemd/default.nix

@@ -56,7 +56,7 @@ in stdenv.mkDerivation {
   #dontAddPrefix = true;
 
   mesonFlags = [
-    "-Ddbuspolicydir=${placeholder "out"}/etc/dbus-1/system.d"
+    "-Ddbuspolicydir=${placeholder "out"}/share/dbus-1/system.d"
     "-Ddbussessionservicedir=${placeholder "out"}/share/dbus-1/services"
     "-Ddbussystemservicedir=${placeholder "out"}/share/dbus-1/system-services"
     "-Dpamconfdir=${placeholder "out"}/etc/pam.d"