Backport r1593615, r1594708 from trunk:

    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.


    Add a pointer to /proc/sys/vm/max_map_count for the guard page feature



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.6.x@1594720 13f79535-47bb-0310-9956-ffa450edef68