Optionally exclude ZONEMD RRs in ldns-compare-zone

[gjherbiet]

As with SOA, ZONEMD resource records may not be meaningful when comparing zones (because of the only difference may be the SOA SERIAL value, leading to different ZONEMD RDATA).

This pull request:

• defines a new function ldns_zone_new_frm_fp_l_e to parse a zone excluding a specified RR type;
• adds a new -Z option to ldns-compare-zone to exclude ZONEMD RR type from zone comparison.

When setting -Z, the last argument to ldns_zone_new_frm_fp_l_e is set to LDNS_RR_TYPE_ZONEMD and this record type is ignored while reading the zone from file.

I know this change could have been implemented purely in examples/ldns-compare-zones.c (e.g. by creating a new empty rr_list, iterating over all the read RRs and only those with a type different from from LDNS_RR_TYPE_ZONEMD) but this seemed inefficient, especially for large zones.

The counterpart is this solution creates an additional "core" function in zone.c, for the sole purpose of this additional option. If this is deemed too obtrusive, I can fallback to the other alternative.

I am also aware that ZONEMD RRs can be filtered out using ldns-read-zone -e ZONEMD but this doubles the amount of parsing and pipes/redirections may not always be available (e. g. while using ldns-compare-zone in systemd Exec* directives).