mount chrootfs read-only at ~/Downloads/Crouton-*

This patch bind-mounts $CHROOT read-only into ~/Downloads/Crouton-$NAME if
we're logged in as a user so that we can access files in the chroot directly
from Chromium OS' "Files" application (e.g. to upload files).

In addition, mounting read-only protects us from the danger of Chromium OS
deleting files randomly from ~/Downloads whenever disk space is low:

  https://support.google.com/chromebook/answer/1061547?hl=en

In fact, even the root user cannot delete files from or modify files within a
read-only mount without first remounting it as read-write!

CAVEAT: I'm assuming that Chromium OS is not so aggressive in freeing up disk
space that it would find and remount this bind-mount as read-write.