This fixes the same problem with uid_t as we had with HAL some time ago
on 64bit architectures in PolicyKit. This time I removed the useless check:

   uid == ((unsigned long) -1)

because this is always false on 64bit (comparison is always false due to
limited range of data type) and because the DBusError from the
dbus_bus_get_unix_user() call is set if the function returns DBUS_UID_UNSET
so we need only to check if the error is set.