Add fueword(9) and casueword(9) functions.  They are like fuword(9)
and casuword(9), but do not mix value read and indication of fault.

I know (or remember) enough assembly to handle x86 and powerpc.  For
arm, mips and sparc64, implement fueword() and casueword() as wrappers
around fuword() and casuword(), which means that the functions cannot
distinguish between -1 and fault.

On architectures where fueword() and casueword() are native, implement
fuword() and casuword() using fueword() and casuword(), to reduce
assembly code duplication.

Sponsored by:	The FreeBSD Foundation
Tested by:	pho
MFC after:	2 weeks (ia64 needs treating)
Replace some calls to fuword() by fueword() with proper error checking.

Sponsored by:	The FreeBSD Foundation
Tested by:	pho
MFC after:	3 weeks
Convert kern_umtx.c to use fueword() and casueword().

Also fix some mishandling of suword(9) errors as errno, which resulted
in spurious ERESTART.

Sponsored by:	The FreeBSD Foundation
Tested by:	pho
MFC after:	3 weeks
Fix atf syntax