support automatic detection and configuration

[aadityabhatia]

Client: detect proxies on LAN before downloading packages. See auto-apt-proxy for a debian-based example.

Server: use avahi to announce proxy server. apt-cacher-ng package on debian includes the following avahi service definition in /etc/avahi/services/apt-cacher-ng.service.

<?xml version="1.0" standalone='no'?>
<!--
From: Lisandro Damián Nicanor Pérez Meyer <[email protected]>
X-Source: http://blog.surgut.co.uk/2013/03/avahi-apt-cacher-ng-sbuild.html
 See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=704790 for details.
-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
 <name replace-wildcards="yes">apt-cacher-ng proxy on %h</name>
 <service protocol="ipv6">
  <type>_apt_proxy._tcp</type>
  <port>3142</port>
 </service>
 <service protocol="ipv4">
  <type>_apt_proxy._tcp</type>
  <port>3142</port>
 </service>
</service-group>