Add two new macros DRIVER_MODULE_ORDERED() and EARLY_DRIVER_MODULE_ORDERED()
that allow a module to use an order other than the default of
SI_ORDER_MIDDLE when registering a driver. This can be useful for drivers
in a kld that contain multiple new-bus drivers as part of one logical device
driver. A typical case would be to use SI_ORDER_LAST for the "main" driver
to ensure that any other "helper" drivers are registered and available
before the "main" driver attempts to attach.

Tested by: kib, Jason Harmening jason harmening / gmail
Approved by: re (kib)
MFC after: 1 week