Add type qualifier volatile to the base (userspace) address argument
of fuword(9) and suword(9).  This makes the functions type-compatible
with volatile objects and does not require devolatile force, e.g. in
kern_umtx.c.

Requested by:	bde
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks
Clean up the types of a few strings to make them const when they are never
written to.