Factor out some clutter in dnsbl.lua

The constructor for DNS blacklist service objects was identical in every
case. Factoring it out will reduce errors and complexity.
Update the usage examples in match.lua

The example uses of nmap.receive_buf() were missing the second
parameter, keeppattern. Also referenced the match library from
nmap.receive_buf's NSEdoc to avoid reimplementations of buffered
sockets.