Initial support for active FTP downloads via the FTP PORT command.

Initial support for active FTP downloads via the FTP PORT command.

Squid accepts PORT command on the client side, but still uses passive transfer
on the server side. The PORT command response is not sent to the client until
the server-side PASV command succeeds. The data connection to the client is
not opened until Squid receives the RETR command from the client.

Squid requires either PORT or PASV command before data transfers. RFC 959 says
PORT is optional because default ports can be used.

RFC 959 also seems to imply that Squid should originate active connections to
client from port 20.  The code to do that is commented out for now because it
would prevent support for concurrent data connections. The code configuring
this outgoing (but to-client) connection may need more work as we do a lot
more for outgoing to-server connections.

Active data upload has not been tested.