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.c

@@ -46,3 +46,16 @@ void ask_password_agent_close(void) {
         (void) wait_for_terminate(agent_pid, NULL);
         agent_pid = 0;
 }
+
+int ask_password_agent_open_if_enabled(BusTransport transport, bool ask_password) {
+
+        /* Open the ask password agent as a child process if necessary */
+
+        if (transport != BUS_TRANSPORT_LOCAL)
+                return 0;
+
+        if (!ask_password)
+                return 0;
+
+        return ask_password_agent_open();
+}

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);