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 6d8a514ecbd72d1f8b2b12fbbbca5c5f87085abd)