Added it so that the client generates or reads a "password" and passes
it to the server. The server then checks if it exists in a file and then
establishes a connection if the key is in a list of known keys or if the
user manually accepts the connection. If the latter is done, then it
adds the key for that client into the list of known keys.

Problems: the selection for wether the user accepts or rejects a
connection is handled in threading, so the input may not register
imediatly. I don't which thread has control over input.