Proper ssh knownhosts hostkey checking

[kke]

Traditional known_hosts checking:

1. if h.HostKey is set, use a fixed hostkey checker (should deprecate)
2. If SSH_KNOWN_HOSTS is set, use it as path. If it's set to /dev/null or "", use InsecureIgnoreHostKey
3. If ssh_config returns something for UserKnownHostsFile, use it as path
4. Fall back to ~/.ssh/known_hosts2

When a hostkey is not found from the known hosts file, add it instead of returning an error. Could probably later add some switch for "strict hostkey checking".

As a sidenote, it's slightly amazing that all this is expected to be figured out / done by everyone on their own when using the crypto/ssh library.