run/mount/systemctl: don't fork off PolicyKit/ask-pw agent when in --user mode

When we are in --user mode there's no point in doing PolicyKit/ask-pw
because both of these systems are only used by system-level services.
Let's disable the two agents for that automaticlly hence.

Prompted by: #20576

src/systemctl/systemctl.c

@@ -925,6 +925,11 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                         assert_not_reached();
                 }
 
+        /* If we are in --user mode, there's no point in talking to PolicyKit or the infra to query system
+         * passwords */
+        if (arg_scope != UNIT_FILE_SYSTEM)
+                arg_ask_password = false;
+
         if (arg_transport == BUS_TRANSPORT_REMOTE && arg_scope != UNIT_FILE_SYSTEM)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                        "Cannot access user instance remotely.");