more debugging updates

src/alloc-slab.c: Switch free slab list from a stack to queue, and
make sure there are SLAB_RESERVE in the queue before an old slab is
dereferenced.  This causes a fault when dereferencing stale pointers
to the last SLAB_RESERVE full slabs.

src/log.c: Make struct logEntry *last static, to make sure gcc does
not optimize it away.
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-13