Treat records below a DNAME as out-of-zone data

Treat records below a DNAME as out-of-zone data

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 75c0d85fc48de8a456d47dd03b7355eac55db1f0)