Avoid segfaulting when maildir_prefix is unset.

  This means we can be launched with no configuration file, and
 we don't abort.

  Closes #98.
  If there is no keymap defined allow exiting at least.

  When there is no defined "keymap" for performing input handling
 we'll fake one that just contains four entries:

    q -> exit();
    Q -> exit();
    x -> exit();
    X -> exit();

  This closes #99.