when using tor .onion addresses, skip ssh-keyscan (no proxying support)

Tor has special domain names called ".onion addresses".  They can only be
resolved by using a proxy via tor. While the rest of the openssh suite
fully supports proxying. ssh-keyscan does not, so it can't be used for
.onion addresses because it just barfs saying unknown host
Merge pull request #1287 from eighthave/master

support private, secure shares with tor .onion addresses