Merge the fueword(9) and casueword(9).  In particular,

MFC r273783:
Add fueword(9) and casueword(9) functions.
MFC note: ia64 is handled like arm, with NO_FUEWORD define.

MFC r273784:
Replace some calls to fuword() by fueword() with proper error checking.

MFC r273785:
Convert kern_umtx.c to use fueword() and casueword().
MFC note: the sys__umtx_lock and sys__umtx_unlock syscalls are not
converted, they are removed from HEAD, and not used.  The do_sem2*()
family is not yet merged to stable/10, corresponding chunk will be
merged after do_sem2* are committed.

MFC r273788 (by jkim):
Actually install casuword(9) to fix build.

MFC r273911:
Add type qualifier volatile to the base (userspace) address argument
of fuword(9) and suword(9).