Documentation and debug output for Acquire::http::Proxy-Auto-Detect

[Tim Small <tim@seoss.co.uk>]

I was debugging a problem with proxy auto detection (which I noticed had
stopped working on my box in sid).  I noticed that APT now seems to parse 
the stderr output from the Acquire::http::Proxy-Auto-Detect command which 
I use (ironically it outputs on stderr for debugging purposes), and this
had broken my setup.

Working out what was happenning requried diving into the code, so I've
also created a patch which expands the debugging output to hopefully make
things clearer for others.

I've created a couple of patches (to follow) which (I hope) make the
documentation less ambiguous.