Force ssh on *nix

[wrathematics]

I nominate deprecating plink support outside of windows. It will simplify the internals quite a bit. Macs come with ssh (I think they have to click a check box in finder somewhere, but they should have it), and surely every Linux box has ssh.

I will do this and submit a PR if you agree. If not I can leave it as is.

[snoweye]

I did not assume everyone has ssh. plink and putty do work in unix and mac. I don't have a better way to generate ppk keys except to convert it from ssh key on unix or mac!! Generating a pair of ppk key in windows is tedious.

[snoweye]

The powershell in windows has ssh, so in near future I will say cmd will be replace by powershell. So we don't need to worry this when it happens.