RFC 959 *requires* to implemet a portion of the telnet
protocol in the command channel. Without this ftlib has
problems interacting with RFC959-compliant server (most
Unix ftp daemons are compliant) if, e.g., a directory
or a filename contains russian characters.

The attched patch implements a minimal requirement (it
doubles chr(255) in the .putline() method), but it
allows a programer to choose if a session will be
strict or not. Default value is "not strict" because
most clients (even Unix clients) do not implemet this
telnet option, and most Windoze servers do not implemet it.

Tested with ProFTPd on GNU/Linux system.

Since I've created the issue I found there are different servers even in
Unix. ProFTPd (and, I believe wu-ftpd) strictly implement
telnet-in-command channel, they even don't have an option to turn it
off. PureFTPd doesn't implement it.

On the client side - lftp (command line client) and squid (well-known
web/ftp proxy) implement telnet-in-command channel.

Other than IAC what other chars need to be doubled?
As an alternative to a brand new TelnetFTP class this can be implemented as a FTP class attribute ("strict_telnet" maybe) defaulting to True.

> Other than IAC what other chars need to be doubled?

Only IAC must be doubled. Also there have to be a special prefix for urgent (out-of-bound) commands (ABORt); I didn't implement that.

> As an alternative to a brand new TelnetFTP class this can be implemented as a FTP class attribute ("strict_telnet" maybe) defaulting to True.

See the first patch. Default value is False to preserve backward compatibility.