dnscrypt-proxy-multi.rb: Validate/Use DNSSEC

[yhaenggi]

2 Feature Requests:

Validate the DNSSEC entries of the domains specified by --resolver-check.
Also an option to only use dnssec aware servers.

[konsolebox]

Maybe we can use dnsruby for this. I'll see what I can do.

[konsolebox]

I added the -z option in e8faf34. It allows only using services that are labelled to support DNSSEC.

I think it would take a while before I decide to add the feature that validates DNSSEC since it complicates the code and adds heavy dependencies. I also noticed that there are only very few domains that support DNSSEC.