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 just hope Chromium OS is not so aggressive in freeing up disk space
that it would first detect and remount this bind-mount as read-write. ;)

TODO:

This bind-mount is currently not unmounted when exiting Crouton.  I
don't know where in "host-bin/unmount-chroot" to place such code.