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.
Apparently I'm not self-consistent.
I think if you're doing just `print $2` you want quoting (no args).  If you're supporting args then you need to change the awk line to grab the rest of the line, and then probably do an `eval "$exec_cmd" &`