DNS: Add POSIX DNS implementation

[0x5ECF4ULT]

What's the problem?

Kerberos.NET doesn't include "autodiscovery" of the KDC on a Linux host. Therefore one must "rely completely on a krb5 configuration file to provide all the necessary records, but that's complicated and messy." (qouted from here)

What's the solution?

I've implemented a native approach using resolv.h and friends in libc. I basically ported code written by Gerald Carter. Sadly, I don't have the original link to the gist anymore.

What issue is this related to, if any?

#185