switchroot: Use MS_MOVE to / rather than chroot
switchroot: Use MS_MOVE to / rather than chroot

This fixes a number of bugs; the two biggest are:

1) On shutdown systemd can correctly umount /
2) We can use linux-user-chroot inside the system, as is
   necessary to do upgrades

See http://lists.freedesktop.org/archives/systemd-devel/2012-September/006703.html

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