add optional whitelisting of fingerprints for connecting

btw, please feel free to lambast this code for style or non-idiomatic stuff.. i'm still learning go...
Probably want this to happen in the auth phase? See `PublicKeyCallback` (already doing something similar with checking bans).

I'd pull it out into a helper member, like IsWhitelisted or something.

Probably better to use a map lookup rather than looping (similar to bannedPk).