virtlockd: improve systemd units

virtlockd: improve systemd units

- Pass VIRTLOCKD_ARGS through to virtlockd.

- Use SIGUSR1, not SIGHUP, in ExecReload. At present, virtlockd only
  responds to the former.

- Have "systemctl enable virtlockd.service" enable virtlockd.socket,
  rather than throw an error.

- Make virtlockd.socket wanted by sockets.target, rather than
  multi-user.target. This is consistent with other socket units in
  Fedora, and it ensures that the socket is available before libvirtd is
  started.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>