In some cases, setting qctx->result to DNS_R_SERVFAIL causes the value
of a 'result' variable containing a more specific failure reason to be
effectively discarded.  This may cause certain query error log messages
to lack specificity despite a more accurate problem cause being
determined during query processing.

In other cases, qctx->result is set to DNS_R_SERVFAIL even though a more
specific error (e.g. ISC_R_NOMEMORY) could be explicitly indicated.

Since the response message's RCODE is derived from qctx->result using
dns_result_torcode(), which handles a number of possible isc_result_t
values and returns SERVFAIL for anything not explicitly listed, it is
fine to set qctx->result to something more specific than DNS_R_SERVFAIL
(in fact, this is already being done in a few cases).  Modify most
QUERY_ERROR() calls so that qctx->result is set to a more specific error
code when possible.  Adjust query_error() so that statistics are still
calculated properly.  Remove the RECURSE_ERROR() macro which was
introduced exactly because qctx->result could be set to DNS_R_SERVFAIL
instead of DNS_R_DUPLICATE or DNS_R_DROP, which need special handling.
Modify dns_sdlz_putrr() so that it returns DNS_R_SERVFAIL when a DLZ
driver returns invalid RDATA, in order to prevent setting RCODE to
FORMERR (which is what dns_result_torcode() translates e.g. DNS_R_SYNTAX
to) while responding authoritatively.