Add proxy auto detect support

[Markus Rexhepi-Lindberg]

We have migrated our Ubuntu desktops to use the APT config option 'Acquire::http::Proxy-Auto-Detect' instead of the 'Acquire::http::Proxy' and this breaks update-manager.

Looking at the 'init_proxy' function it seems that only the 'Acquire::http::Proxy' APT config option is supported for the use of proxy. Can support for the 'Acquire::http::Proxy-Auto-Detect' (as well as 'Acquire::https::Proxy-Auto-Detect' for https) APT config option be added to update-manager?

A workaround that I managed to use is to export the http_proxy and https_proxy environment variables and then run `do-release-upgrade`. Though this has a couple of drawbacks, for instance that the GUI version of update-manager is not possible to use. I'm just mentioning this if someone else is stuck while using 'Proxy-Auto-Detect' for APT.

[Julian Andres Klode (juliank)]

There are quite a few places in Ubuntu that have badly adapted apt proxy settings. I think aptdaemon, PackageKit are affected as well. It's not possible to correctly reimplement apt proxy settings by reading configuration, files need to be fetched by apt's acquire library.

[Julian Andres Klode (juliank)]

Oh well, seems aptdaemon and PackageKit are fine. But I do vaguely recall other bits being affected too.

[Brian Murray (brian-murray)]

The Groovy Gorilla has reached end of life, so this bug will not be fixed for that release