unity: set env variables for global menu

Fixes #869.
unity: fix Unity HUD (alt key menu)

Fixes #802.
unity: move hacks into profiled and autostart
unity: quote window-stack-bridge
unity: code review fixes
unity: unify common service launching code
unity: remove useless local in execfromconf
Merge pull request #900 from smibarber/fix-global-menu

Fix Unity global menu and HUD
use ~/Downloads as home directory for chroot user

This patch mounts your ~/Downloads directory as the home directory for
the main user in all of your Crouton chroots.  By doing this, you get:

* easy access to files in/beneath your home directory from Chromium OS
* an encrypted home directory because ~/Downloads is already encrypted
* the same, persistent home directory across different Crouton chroots