Rearrange slab allocation header; minor slab fixes.

src/alloc-slab.c (ALLOC_MAGIC, FREE_MAGIC): Replace with single-byte values.
(struct alloc_header): Move file_id and line into second 32-bit word
of header.
(slab_alloc): Remove commented-out debug statements.
(slab_unalloc): Remove memset() of freed block and commented-out debug
statements.
(slab_free): memset() freed blocks here instead.  Try to fix
*_alloc_size counting errors (use the originally requested length
rather than rounded-up length)
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-15