> Thinking back about this, I wonder whether "FTPS" could be a better name to use instead of "FTP_TLS".
> It's shorter, easier to remember, and also makes more sense since also SSL can be used, not only TLS.

What do you mean by "also SSL can be used"?
Wikipedia has an interesting article about the subject:
http://en.wikipedia.org/wiki/FTPS

Secured FTP with explicit negotiation (what we are doing) is sometimes
called "FTPES" (that's how it's named in FileZilla indeed).
"FTPS" is more often used to describe secured FTP with implicit
negotiation, i.e. the SSL session is established before the FTP protocol
even kicks in.

I think FTP_TLS is a fine name. Perhaps we can simply make the above
distinction clearer in the docs.