desktop: install steam
desktop/drivers: add i386 drivers
desktop: remove steam apt repo, get from nonfree

Dara points out that, even though the official downloadable Steam
installer adds the repo.steampowered.com apt repo, the installer is
actually packaged in Debian jessie nonfree. So we'll just use that.

It's a bit out-of-date already, but the binary basically just downloads
a few hundred MB of the latest Steam installer. So it's not really a
concern if the downloader is out-of-date.
desktop/steam: install from local package

There's a bug (Debian bug#772598) preventing preseeding acceptance of
license answers in the current steam package.

We use a custom-built package with my patch to avoid that. We can remove
a lot of the crappy exec code once we switch to using apt (either
because we finally set up our own apt repo, or because the bug gets
fixed in Debian).
commnon/apt: trigger apt update after adding i386
desktop/sshfs: install libpam-mount via dpkg