Introduce DNS block list in dnscache(8)

Introduced support for a DNS block list in dnscache(8) resolver.
DNS block list is a list of domain names which are to be blocked
by the resolver. Client requests querying for such domain names
are dropped by the resolver. This would add an additional layer
of security for DNS clients and would also help reduce malicious
traffic.

DNS block list is a 'cdb' database created using tinydns-data(1).
tinydns-data(1) creates the 'cdb' database by reading a 'data' file.
User should list the malicious domain names into this 'data' file,
as generic domain records, one on each line, as:

    :bad.domain.com:284::::

Number '284' is not used, it can be anything > 255. tinydns-data(1)
would create a 'data.cdb' database from the 'data' file. Rename this
'data.cdb' to 'dnsbl.cdb', for that is the file read by dnscache(8)
resolver.

    $ mv data.cdb dnsbl.cdb

dnscache(8) would read 'dnsbl.cdb' from its working($ROOT) directory
defined by the /etc/ndjbdns/dnscache.conf file.
New error code - error_blockedbydbl

Defined a new error code 'error_blockedbydbl' to identify the
queries that were dropped by the dnscache(8) resolver. The resolver
logs this error code along with an error message, for each dropped
(or blocked) client request.