Add support for serving as a Nagios service check

[atc0005]

Perform query against all DNS servers in /etc/resolv.conf and return pass/fail result.

refs #4

[atc0005]

Or, take a list of DNS servers (or config file) and have Nagios check the current host for a matching record in all specified DNS servers.

Bonus: allow a specific DNS server to return a response that the record does not exist (e.g., internal host record that should not exist in public DNS server.

[atc0005]

I think the best approach here is to create a new binary instead of trying to fit a nagios-specific flag into the current cmd/dnsc binary.

If I go this route, the atc0005/check-vmware project can serve as an example of providing flags specific to a specific binary. This way the same config package can be used by both binaries, but only the flags intended for a specific binary will be exposed (while flags not specific to a binary will be seen by both).