os probing

Joshk and I got to work on os-prober today, and d-i will now detect existing DOS, Windows, freeDOS, hurd, and linux installs, and properly set up at least grub chain loading for the DOS ones. Will download a Hurd image tonight and get it booting too. Properly booting other linux distros will take more work. Actually, all I've been able to test so far is freeDOS; installing any of the others is too much work.