MFC r345363 (by imp): Make WD and WDC aliases for HGST.

HGST was bought by WDC. Over the years, it has sold different drives
branded as HGST, WD or WDC. All of them need the HGST workaround of
sending 4k-sized packets (or multiples of 4k). And the ones that don't
really need this aren't broken by this change. Submitter is the vendor
who has tested these changes on a number of drives. I've simplified it
slightly, since we don't need additional vendors for this at this
time.