Support for apt-proxy

[fpletz]

• apt-proxy can also advertise itself using avahi (see BTS bug #411355)
• apt-zeroconf should allow detecting that proxy -> this allow apt autoconfiguration for laptop users when changing networks
• if apt-zeroconf is only used to detect apt-proxy instances, use it if one available, else try default locations
• if apt-zeroconf can also connect other computers, and the file is not available on the local network, use the apt-proxy instance to get the file downloaded (that will also benefit people not using apt-zeroconf)
• other situation, act like now

[fpletz]

Deferred until apt-proxy has support for avahi.

[anonymous]

this is very similar to bug #28 and could potentially solve it.