switch default zone database from "qp" to "qpzone"

switch default zone database from "qp" to "qpzone"

use the dns_qpmulti-based "qpzone" by default throughout BIND,
instead of the existing dns_qp-based "qp", when creating zone
databases. (cache databases still use "qp".)

the "--with-zonedb" option has been updated in configure.ac to permit
the use of both "qp" and "qpzone" databases.

in zone.c there was a test that prevented any database type other than
"qp" from hosting an RPZ. this was outdated, and has been removed.