Make miibus(4) fully ifnet(9) agnostic, removing layering violation.

This allows to mii_attach() in drivers prior to if_attach(), but on the
other hand puts some extra work on drivers.

Drivers now should:

• Do mii_attach() before if_attach().
• Do baudrate management theirselves in miibus_statchg devmethod.
• Do link state management theirselves in miibus_linkchg devmethod.

Some PHYs require to obtain MTU of the interface. Instead of doing it
via ifnet(9) layer, provide new miibus method miibus_readvar. Drivers
that expect such PHYs must implement this method and return IF_MTU.

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