Ftp gateway: initial implementation.

Ftp gateway: initial implementation.

The first client command must be USER with
<username>@<hostname>[:<port>] parameter.  Origin server connection is
bound to client connection.  After origin server connection is
established, it cannot be changed, i.e. client cannot connect to
another origin server.  Once server or client connection is closed,
the other side should be closed as well.

Only passive mode is supported on both client and server side.  IPv6
(RFC 2428 EPSV command) is not supported.

Implemented data transfer commands: RETR, LIST, NLST.  Data push
commands (STOR, APPE) are not implemented.

Many rough edges: aborts/timeouts/error handling, logging and more.