Test and fix network login

The server-side credentials checking was overhauled. Nonetheless, twisted's
scheme does not seem to be particularly secure, since it uses MD5 in a
challenge-response scheme. Temporarily, the pbkdf2 hash is now used as
"password" but this means that the information stored on the server-side is
sufficient to authenticate as a user.

Besides some bugs on the client-side updater were fixed.