auto-login ftp URL is used when $ftp_proxy is defined. It now prints:
Auto-login using ftp URLs isn't supported when using $ftp_proxy
Should solve rest of [bin/3643].
Whilst this is inconsistant with the behaviour when $ftp_proxy isn't
defined, the following constraints apply:
* it's not possible to support ftp URL auto-login when $ftp_proxy is
defined, since it uses http not ftp, and you can't `login' to http
servers; fudging this would require a major rewrite of ftp anyway)
* silently ignoring $ftp_proxy and not using it if an ftp auto-login
URL is given is bad user interface design)
* mrg & others will harrass me if I remove support for autologin ftp URLs
when $ftp_proxy isn't defined, even though it made the behaviour
consistant whether $ftp_proxy was set or not.