systemd: Add support for path units.

This allows to define systemd.path(5) units, for example like this:

{
  systemd = let
    description = "Set Key Permissions for xyz.key";
  in {
    paths.set-key-perms = {
      inherit description;
      before = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      pathConfig.PathChanged = "/run/keys/xyz.key";
    };

    services.set-key-perms = {
      inherit description;
      serviceConfig.Type = "oneshot";
      script = "chown myspecialkeyuser /run/keys/xyz.key";
    };
  };
}

The example here is actually useful in order to set permissions for the
NixOps keys target to ensure those permisisons aren't reset whenever the
key file is reuploaded.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>