/nix bind mounted from a fs not rootfs.
As niksnut found, if the sources of bind mounted directories are
umounted before remounting them as "ro", a "mount -o remount,ro" on the
bind targets will not leave the filesystem properly umounted, the same
way as a "-o remount,ro" would do on a usual filesystem.
So, the trick of always remounting as 'ro' before attempting an umount seems
to fix the problem.
svn path=/nixos/trunk/; revision=19008
483ee6219e