DNS Update fix. A misconfigured server could crash during DNS update

DNS Update fix. A misconfigured server could crash during DNS update
processing if the configuration included overlapping pools or
multiple fixed-address entries for a single address.  This issue
affected both IPv4 and IPv6. The fix allows a server to detect such
conditions, provides the user with extra information and recommended
steps to fix the problem.  If the user enables the appropriate option
in site.h then server will be terminated