dbus: check whether the client wanted interactive authentication

Some clients do not want to prompt for passwords, e.g. when authorising keyboards
when the screen is locked, prompting for a password doesn't make sense because
the user cannot enter anything.
This change extracts the flags and checks whether the client called with
G_DBUS_CALL_FLAGS_ALLOW_INTERACTIVE_AUTHORIZATION.

src/DBus/DBusBridge.cpp

@@ -477,7 +477,11 @@ namespace usbguard
     polkit_details_insert (details, "polkit.message", "This USBGuard action needs authorization");
     USBGUARD_LOG(Trace) << "Customized.";
     USBGUARD_LOG(Trace) << "Checking authorization of action \"" << action_id.str() << "\" with Polkit ...";
-    const PolkitCheckAuthorizationFlags flags = POLKIT_CHECK_AUTHORIZATION_FLAGS_ALLOW_USER_INTERACTION;
+    GDBusMessage *message = g_dbus_method_invocation_get_message (invocation);
+    const PolkitCheckAuthorizationFlags flags = (g_dbus_message_get_flags (message) & G_DBUS_MESSAGE_FLAGS_ALLOW_INTERACTIVE_AUTHORIZATION)
+        ? POLKIT_CHECK_AUTHORIZATION_FLAGS_ALLOW_USER_INTERACTION
+        : POLKIT_CHECK_AUTHORIZATION_FLAGS_NONE;
+
     PolkitAuthorizationResult* const result = polkit_authority_check_authorization_sync
       (authority,
         subject,