(lwpCreate): Check STKALIGN is sane.

(lwpCreate): Check STKALIGN is sane.

(lwpCreate): Used to allocate one byte more than necessary.  The fix
won't save memory (malloc() will add the byte back), it's just clearer
this way.

(lwpCreate): Clean up computation of sp.  Rename parameter size to
stacksz.

(lwpCreate): Always compute newp->lowmark and newp->highmark from sp,
not from s.  Old code misaligned newp->himark for stacks growing
upward when stack size argument wasn't aligned.