Fix dropped covers field for SIG records in dns_diff_apply

rdata_covers() in lib/dns/diff.c discriminated only on
dns_rdatatype_rrsig (46) and returned 0 for the legacy SIG (24), so
the covered-type field was silently discarded on the dynamic-update
and IXFR paths.  Every SIG rdataset was then filed in the zone DB
under typepair (SIG, 0) instead of (SIG, covered_type); a second SIG
add with a different covers but a different TTL collided at that
bucket, tripped DNS_DBADD_EXACTTTL in qpzone, returned
DNS_R_NOTEXACT, and came back to the client as SERVFAIL.

Use dns_rdatatype_issig() here so both SIG and RRSIG carry their
covers through the diff, matching the helper pattern already used in
lib/dns/master.c, lib/ns/xfrout.c, lib/dns/qpcache.c, and the
dns__db_findrdataset() REQUIRE that the surrounding merge request
just relaxed.

(cherry picked from commit 0a5ba57116)

lib/dns/diff.c

@@ -40,7 +40,13 @@
 
 static dns_rdatatype_t
 rdata_covers(dns_rdata_t *rdata) {
-	return rdata->type == dns_rdatatype_rrsig ? dns_rdata_covers(rdata) : 0;
+	if (rdata->type == dns_rdatatype_rrsig ||
+	    rdata->type == dns_rdatatype_sig)
+	{
+		return dns_rdata_covers(rdata);
+	}
+
+	return 0;
 }
 
 isc_result_t