Stop supporting fallback between SSH versions.

Stop supporting fallback between SSH versions.

The UI now only has "1" and "2" options for SSH protocol version, which
behave like the old "1 only" and "2 only" options; old
SSH-N-with-fallback settings are interpreted as SSH-N-only.

This prevents any attempt at a protocol downgrade attack.
Most users should see no difference; those poor souls who still have to
work with SSH-1 equipment now have to explicitly opt in.