Make g_access() KASSERT() more useful.

Sponsored by:	EMC / Isilon Storage Division
Obtained from:	Isilon OneFS
MFC after:	2 weeks
Stop treating LUN 0 as mandatory.  There is no reason to do that.

Suggested by:	mav@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Add support for emulating the byte move and sign extend instructions:
"movsx r/m8, r32" and "movsx r/m8, r64".

Approved by:	grehan (co-mentor)