Greatly enhance the slab allocator.

Greatly enhance the slab allocator.

configure.in: Look for mprotect() function.
src/Makefile.am: Add slab-read program.
src/slab-read.c: New helper program.
src/alloc-slab.c (SLAB_DEBUG_HEADER): New bitmask macro.
  (SLAB_DEBUG_LOG): Likewise.  Implement its functions.
  (SLAB_DEBUG_PERMS): Likewise.  Implement its functions.
  (SLAB_DEBUG): Update default.
  (slab_alloc): Call slab_unprotect() when pulling free_slab_head.  Call
    slab_log_alloc() on any slab allocation.
  (slab_unalloc): Call slab_log_free() on slab unallocation.  Call
    slab_unprotect() and slab_protect() when manipulating free slabs.
  (slab_malloc): Call slab_log_alloc() on large object allocation.
  (slab_free): Call slab_log_unmap() on large object free.