DNAME records indicate bottom of zone and thus no records below a DNAME
should be DNSSEC-signed or included in NSEC(3) chains. Add a helper
function, has_dname(), for detecting DNAME records at a given node.
Prevent signing DNAME-obscured records. Check that DNAME-obscured
records are not signed.
(cherry picked from commit 75c0d85fc4)