Fix the NA and PD allocation code to handle the case where a client

Fix the NA and PD allocation code to handle the case where a client
provides a preference and the server doesn't have any addresses or
prefixes available.  Previoulsy the server ignored the request with
this patch it replies with a NoAddrsAvail or NoPrefixAvai respone.
By default the code performs according to the errata of August 2010
for RFC 3315 section 17.2.2, to enable the previous style see the
seciton on RFC3315_PRE_ERRATA_2010_08 in includes/site.h.  This option
may be removed in the future.
Thanks to Jiri Popelka at Red Hat for the patch.
[ISC-Bugs #22676]