Test and debug the RAID 6 implementation in RAIDframe.

NetBSD uses RAIDframe (raid(4)) and stub code exists for RAID6 but is not very documented or well tested. Obviously, this code needs to be researched and vetted by an interested party. Other BSD projects should be consulted freely.

Milestones: * setup a working RAID 6 using RAIDFrame * document RAID6 in raid(4) manual page * port/develop a set of reliability and performance tests * fix bugs along the way * automate RAID testing in atf

Bonus: * Document how to add new RAID levels to RAIDframe * (you're awesome bonus) add RAID 1+0, etc