Misc fixes

A couple of changes here and there:

* Debian/Ubuntu machines need `libsdl1.2-dev` installed; this was not listed in the README
* Added a `prepare-debian-ubuntu.sh` script that installs all necessary packages, and fixes `/bin/sh`, similar to `prepare-mac-os.sh`
* Unpacking archives now gives proper error codes instead of crashing, which effectively fixes redownloading of broken archives (it will reattempt the download, instead of blowing up)
* Unpacking a tar archive as root when the archive is located on a remote filesystem, and owned by a locally nonexistent UID, no longer spews errors for every file in the archive...
* Removed `wget`'s progress bar, as it causes trouble with a non-TTY stderr - I'm working on something that needs this to work

If the last one is problematic, it wouldn't be too much trouble to make a wget wrapper that dynamically enables the progress bar depending on if stderr is a TTY or not, but it seemed a bit overkill - adding an extra flag is a much smaller diff.