dns/rbldnsd: update 0.997a -> 0.998

• use ${ETCDIR}
• don't install RPM spec and debian packege spec - they are no use on FreeBSD
• install example zone file with default permissions (readable to all users) there is no need to limit access to examples

PR: 214426
Changes: http://www.corpit.ru/mjt/rbldnsd/NEWS
Submitted by: Anton Yuzhaninov <citrin+pr@citrin.ru>
Approved by: Alexey Degtyarev <alexey@renatasystems.org> (maintainer)