This reverts commits
d72238fcb34abc81aca97c5fb15888708ee937d3 and
f3accc08.
OLPC runs / as a bind-mount, so this must be remounted RO during
shutdown to avoid corruption.
As Lennert can't recall the exact reasons for making the shutdown
code skip bind mounts, revert to previous behaviour to solve the
issue for OLPC.
http://lists.freedesktop.org/archives/systemd-devel/2012-April/004957.html