Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Consistently set us to be SPC3 for our inquiry data.

For mptest, add delays to I/O that simulate real disks better. This
also allows us to simulate what happens when a device goes away
with active transactions. It's pretty spectacular.

Sponsored by: Panasas
MFC after: 1 month

Add a multipath oriented test.