Bug#54524: http_proxy and web clients.
This is already standard, but I think it should be into policy because I
already saw some programs deviating from this expected behaviour.
Web clients should default to try to fetch URLs by a direct connection to
the target host. If the `http_proxy' variable is defined, it should specify
an URL that would be used as a proxy. Programs should not try to access an
URL directly if this variable is defined. Programs that have the ability to
do it should also respect the ftp_proxy variable, but not all ftp clients
are forced to fo that, since it needs handling of the HTTP protocol.