Support the bootloader's single 16-bit 'outw' access to the Divisor
Latch MSB and LSB registers.

Approved by:	neel (co-mentor)
* Provide information in error causes in ASCII instead of
  proprietary binary format.
* Add support for a diagnostic information error cause.
  The code is sysctlable and the default is 0, which
  means it is not sent.

This is joint work with rrs@.

MFC after: 1 week
Though there currently isn't a way to insert new media into an ATAPI
drive, at least pretend to support Asynchronous Notification (AN) to
avoid a guest needlessly polling for it.

Approved by:	grehan (co-mentor)