support private, secure shares with tor .onion addresses

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.

This patch skips the ssh-keyscan check if the domain name ends with `.onion`, which will only happen if the user if using  a tor onion address.

Supporting tor makes sparkleshare the ultimate private, secure file sharing system.  dropbox supports tor because it has manual proxy settings,