Document our guard areas, and make them optional.

(OVERHEAD): New macro, that computes a size including the guard
	area overhead.  Two versions, depending on if
	USE_MALLOC_GUARDS is defined or not.
(smalloc): Reindent.  Test USE_MALLOC_GUARDS.
(sfree): Test USE_MALLOC_GUARDS.
(srealloc): Test USE_MALLOC_GUARDS.