ZONEMD needs to be able to digest SIG and RRSIG records. The signer
field can be compressed in SIG so we need to call dns_name_digest().
While for RRSIG the records the signer field is not compressed the
canonical form has the signer field downcased (RFC 4034, 6.2). This
also implies that compare_rrsig needs to downcase the signer field
during comparison.
(cherry picked from commit 006c5990ce)
Mark Andrews
54c89f75f3