layer/validate: disable explicit RRSIG query if RRSIG for DS is missed
Relates to #372 (closed) - forwarding mode.
Previously if an misconfigured upstream returned DS without RRSIG the kresd started a loop to ask for RRSIGs.
Even if RRSIG for DS exists, upstream will not return it because DS is parent-side RR so this query is meaningless.
Edited by Petr Špaček