macos: do not call dbus for uniqueness

DBus can cause instability on the MacOS platform, and regardless
MacOS handles application uniqueness already.

The batch version has also been modified as it only applies if
compiled as an application, since it returns FALSE if it is complied
as a console app.

(cherry picked from commit 9102e47637)

app/unique.c

@@ -48,6 +48,8 @@ gimp_unique_open (const gchar **filenames,
 {
 #ifdef G_OS_WIN32
   return gimp_unique_win32_open (filenames, as_new);
+#elif defined (PLATFORM_OSX)
+  return FALSE;
 #else
   return gimp_unique_dbus_open (filenames, as_new);
 #endif
@@ -60,6 +62,8 @@ gimp_unique_batch_run (const gchar  *batch_interpreter,
 #ifdef G_OS_WIN32
   g_printerr ("Batch commands cannot be run in existing instance in Win32.\n");
   return FALSE;
+#elif defined (PLATFORM_OSX)
+  return FALSE;
 #else
   return gimp_unique_dbus_batch_run (batch_interpreter,
                                      batch_commands);