Fix assertion failure in dns_db_findrdataset() for SIG records

Fix assertion failure in dns_db_findrdataset() for SIG records

dns__db_findrdataset() had a REQUIRE() that only accepted
dns_rdatatype_rrsig when the covers parameter was set.  A dynamic
update containing a SIG record (type 24) would trigger this
assertion, crashing named.  Use dns_rdatatype_issig() to accept
both SIG and RRSIG.

(cherry picked from commit 03edeccaa16991e05c101ae82a03568ae1b2dbd0)