Add support for NSEC3.

NSEC3PARAM is loaded from NSEC3PARAMRecord attribute at zone apex
in LDAP. Default (when the attribute is not present) is to use NSEC.

Limitation: Only one NSEC3PARAM record per zone is supported at the moment.

https://fedorahosted.org/bind-dyndb-ldap/ticket/56

Signed-off-by: Petr Spacek <pspacek@redhat.com>