cryptsetup-generator: imply x-initrd.attach for "usr" and "root" volumes

cryptsetup-generator: imply x-initrd.attach for "usr" and "root" volumes

Let's imply "x-initrd.attach" for "usr" and "root" volumes, so that
we do not attempt to umount them anymore during shutdown.

The names of these volumes have been mandated by the Discoverable
Partition Spec:

https://uapi-group.org/specifications/specs/discoverable_partitions_specification/#suggested-mode-of-operation

Hence it appears reasonably safe to special case these volume names.

Note that a similar logic is implemented in fstab-generator and in fact
PID 1 to treat the root mount and /usr/ mount specially too, to avoid
trying to umount it at shutdown. (This is what fstab_is_extrinsic()
checks).

This should ensure that if /usr/ or / is for some reason a LUKS medium
we won't try to detach it during runtime, which likely fails, since we
run off it.

Note this also moves an ordering dep towards umount.target under the
x-initrd.attach check, becasue that's where the crucial conflicts dep is
placed too.