ask-password-agent: introduce ask_password_agent_open_if_enabled()

This makes the ask-password agent handling more alike the polkit agent
handling again, and introduces ask_password_agent_open_if_enabled() that
works just like the already existing polkit_agent_open_if_enabled().

src/shared/spawn-ask-password-agent.h

@@ -1,5 +1,11 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <stdbool.h>
+
+#include "bus-util.h"
+
 int ask_password_agent_open(void);
 void ask_password_agent_close(void);
+
+int ask_password_agent_open_if_enabled(BusTransport transport, bool ask_password);