ake ftp aware of authentication config.

 bzrlib/transport/ftp.py:
FtpTransport._create_connection): Try the authentication config
efore prompting the user.

 bzrlib/tests/test_ftp_transport.py:
TestFTPServerUI.test_prompt_for_password): New test.

 bzrlib/tests/test_config.py:
TestConfigPath.test_authentication_config_filename): Add test,
ot tested on windows, but should work.

 bzrlib/tests/HTTPTestUtil.py:
AuthServer.authorized): Fix typo.

 bzrlib/config.py:
AuthenticationConfig.__init__): Track the filename getter, we
eed it for _save().
AuthenticationConfig._save): Needed for tests.
AuthenticationConfig._set_option): Basic setter, needed for
ests.
AuthenticationConfig.get_credentials): Rework 'port' acquisition.