Add option to use guard pages

Add new --enable-allocator-guard-pages configure option which works like
--enable-allocator-uses-mmap, but will also add inaccessible guard pages before
and after each memnode.  This will result in higher ressource usage but allow
to find/protect against certain buffer overflow/overread bugs.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1593615 13f79535-47bb-0310-9956-ffa450edef68