PR1623: check_dns: Add --expect-nxdomain

[Jonny007-MKD]

Hi,

Thank you for all these check plugins, they are great!

Yesterday I started to miss one feature: I want to check that certain
domains are blocked by my server and return a NXDOMAIN (that's how my
adblock script configures my unbound instance). I added this option to
check_dns. Please have a look at this pull request:
https://github.com/monitoring-plugins/monitoring-plugins/pull/1623

Best regards,
Jonny007