user detection

[bretth]

It is useful to determine whether a user exists on the server, without storing creation state on the client side. For instance we can test whether root has been disabled by attempting to login with it, or else we could test that a new user has been created by attempting to login with them.

To do this we need to handle a login failure either from first password entry or key. This is a forthcoming feature of Fabric but it may be quicker to just implement it separately.