Fix PrinterSpooler D-Bus signal spoofing

[oubayashi]

The existing policy grants root permission to send messages on the com.redhat.PrinterSpooler interface, but does not deny unprivileged clients. Since the system bus permits broadcast signals by default, an ordinary user can emit a spoofed PrinterSpooler signal despite the comment claiming that only root can send it.

Deny signals on this interface in the default policy and allow them again for root. This preserves reception for all connections and limits the change to signal messages, avoiding an unintended restriction on other message types.