Bug#947811: systemd: don't check *mounted* filesystems by default

[Joshua Hudson]

> If you want to fsck/mount your partitions manually via a custom script
that is run before init, then I would set passno to 0.

I've since masked the job that systemd uses to run fsck. It's still a
bug because if somebody forgets and leaves a filesystem mounted
read-write in the emergency shell (fixing anything other than fsck
problems in the emergency shell usually requires mounting things
read-write), the same problem happens and no custom script is
involved.