Root file system not cleanly unmounted

[Ludovic Courtès]

FYI, if you installed a system from 0.8, chances are that your root file
system is not cleanly unmounted (from what ext4 recovers without any
problems, but still.)

This is fixed by commit 1c27641, so consider running ‘guix pull’ and
‘guix system reconfigure’.

The bug probably appeared as a side-effect of commit cd4a3cb3.
Basically, when rebooting, the devtmpfs file system on /dev would fail
to be unmounted, thus preventing / itself to be properly unmounted.

Thanks,
Ludo’.

PS: Thanks to David for raising it on IRC yesterday.