ssh/auth.c: Handle disconnected sessions

* ssh/auth.c (guile_ssh_userauth_public_key_x)
  (guile_ssh_userauth_public_key_auto_x)
  (guile_ssh_userauth_public_key_try, guile_ssh_userauth_agent_x)
  (guile_ssh_userauth_password_x, guile_ssh_userauth_none_x)
  (guile_ssh_userauth_get_list): Throw `guile-ssh-error' if the session
  is not connected.
* doc/api-auth.texi (Auth): Update.
* NEWS: Update.