About a year ago I found myself fighting a broken FTP server that
couldn't handle multiple passive data transfers through a firewall or
NATed connection. Thankfully, this same problem server supports block
transmission mode, which allows a client to create a single data
connection for transferring multiple files.

I've attached a patch (and sample debug output) for the latest trunk.

Might this be useful to anyone else in any way? I realize any MODE
option rather than S is widely unsupported and possibly an edge case,
but getting this into trunk would be preferable. We've been running this
code under Python2.3 for nearly a year -- the jobs run several times per
hour -- and are extremely happy with the results.