cleanup unused -dev packages with autoremove

[hamish]

Hi,

for future:

at the end of setdown.sh we should try apt-get autoremove without the --yes flag. This may uninstall a lot more than we intend- any packages which are self-compiled must manually install any libraries they need if they want them to hand around.

Some installers (e.g. pgRouting) are naughty and don't clean up their -dev packages after themselves, leading to a couple of hundred extra MB on the final disc. autoremove would catch those.

Hamish