Even more slab allocator updates.

src/slab-alloc.c (SLAB_DEBUG): Default to on.
(SMALL_CUTOFF): Fix default value (must be a multiple of 4).
(slab_unalloc): Fix slab counting.  When SLAB_RESERVE, allocate the
set of pages in a burst, rather than supplementing them as we unmap.
(slab_realloc): Fix a rather embarassing (and LARGE) memory leak.
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-14