Fall back to login command when others fail

We primarily authenticate using the plain authenticate command. We can
however also use the login command unless the server has told us not to.
Use the login command if plain authentication is not available.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Show dialog when there's no authentication method

It is possible that the server and client accepts to common
authentication method. This indicated in the UI. Show a dialog with a
message about it if that happens.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>