Bug#410878: d-i uses mirror/http/proxy setting when it (probably) shouldn't

[Alexander Perlis <aperlis@math.lsu.edu>]

The bug Roland reported is real, and occurs in 2013 with Debian 6 
installer. There should indeed be a distinction between an APT proxy and 
a general-purpose HTTP proxy. Our APT proxy knows how to serve up files 
related to Debian mirrors, but can't do general-purpose HTTP retrievals 
such as preseed/run files from our PXE server.

Currently mirror/http/proxy affects http_proxy globally, for all 
subsequent calls to wget. Might be better to not do that; instead, on 
each APT-related call to wget, if mirror/http/proxy has a value, then 
temporarily store it in http_proxy, call wget, and then restore 
http_proxy to its prior value.