Add in alloc debugging code which can be used standalone to detect some

uninitialized read, and use-after-free errors.  It can also be combined with
debuggers like efence and Purify.  By default nothing should change.

This change introduces one change to the non-debugging code:

-    blok = new_block(0);
+    blok = new_block(POOL_HDR_BYTES);

This is during make_sub_pool.  Technically speaking, this fixes a bug;
the bug was that make_sub_pool was assuming that
BLOCK_MINALLOC > POOL_HDR_BYTES.  Not an unreasonable assumption ... but
the debugging code sets BLOCK_MINALLOC to 0.

Reviewed by:	Jim Jagielski, Rob Hartill, Martin Kraemer


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79372 13f79535-47bb-0310-9956-ffa450edef68