Fix Unity global menu and HUD

Changes by issue:
#869: `UBUNTU_MENUPROXY=1` and `GTK_MODULES=unity-gtk-module` is sufficient to enable global menus.
#802: The Unity HUD has been broken for a while. In trusty, fixing this requires inhibiting `gnome-settings-daemon` from running. Normally, Upstart would handle launching `unity-settings-daemon`. But since we're not using Upstart, `gnome-settings-daemon` is launched by its .desktop file, which wasn't updated to preclude it from starting in Unity. `window-stack-bridge` also needs to be started.

Other changes:
* `croutonversion -a` prints the chroot architecture.
* Consolidated Unity-related hacks into a couple of places so that scripts and desktop files don't need to be littered everywhere.
> Just found that gcc -print-multiarch will do it.

Yes but gcc is not installed by default...

Looking at the output of `dpkg -L hud`, I see there is an upstart configuration file `/usr/share/upstart/sessions/window-stack-bridge.conf`, maybe you can parse the `exec` line just like you do below with indicators?

Then, for consistency, you could also start `unity-panel-service` in the same fashion.