flyctl config validation improvements

[rubys]

From https://community.fly.io/t/fly-config-in-your-choice-of-formats-toml-yaml-or-json/20017/7 :

I wish flyctl would actually go further and also complain ahead of time about settings combinations that other parts of the Fly infrastructure will consider illogical/invalid:

• overlapping port definitions

• auto-start requested on UDP (which doesn’t support it)