Use /var/run/crouton to mount chroots.

Avoids the need to --make-unbindable, prevents polluting the file system, and
simplifies logic.
Properly unmount old chroots that are still mounted.

Avoids issues with upgrading.
Double-unmount for old-style chroots.
Properly handle unmounted chroots.
Use return instead of continue.
Enforce prefix existence
Remove redundant set +e
Unmount chroots before unmounting the prefix.
Add /media tests.