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

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

Add a new personality to mpt(4) devices to allow userland applications to
perform various operations on a controller. Specifically, for each mpt(4)
device, create a character device in devfs which accepts ioctl requests for
reading and writing configuration pages and performing RAID actions.

MFC after: 1 week
Reviewed by: scottl