nixos/stage-1: Ensure correct ZFS mount options

Consider ZFS filesystems meant to be mounted with zfs.mount(8), e.g.
```
config.fileSystems."/media".options = [ "zfsutil" ];
config.fileSystems."/nix".options = [ "zfsutil" ];
```

`zfsutil` uses dataset properties as mount options such that zfsprops(7)
do not have to be duplicated in fstab(5) entries or manual mount(8)
invocations.

Given the example configuation above, /media is correctly mounted with
`setuid=off` translated into `nosuid`:

```
$ zfs get -Ho value setuid /media
off
$ findmnt -t zfs -no options /media
rw,nosuid,nodev,noexec,noatime,xattr,posixacl
```

/nix however was mounted with default mount(8) options:
```
$ zfs get -Ho value setuid /nix
off
$ findmnt -t zfs -no options /nix
rw,relatime,xattr,noacl
```

This holds true for all other ZFS properties/mount options, including
`exec/[no]exec`, `devices/[no]dev`, `atime/[no]atime`, etc.

/nix is mounted using BusyBox's `mount` during stage 1 init while /media
is mounted later using proper systemd and/or util-linux's `mount`.

Tracing stage 1 init showed that BusyBox never tried to execute
mount.zfs(8) as intended by `zfsutil`.

Replacing it with util-linux's `mount` and adding the mount helper
showed attempts to execute mount.zfs(8).

Ensure ZFS filesystems are mounted with correct options iff `zfsutil` is
used.

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