Right now bin/dist_get trusts whatever DNS resolver is active.
This change enables it to verify what it downloads.
Verification itself remains TODO.

This will work best with a unixfs-hash tool which is TODO too.
It'd just do the equivalent of `ipfs add -n -r <dirOrFile>`.

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>