A shellcheck regression quoted `HARD` in
```shell
kill "$HARD" "$pid" > /dev/null 2>&1
```
which would error on an empty "HARD".
Instead of fixing this, use `pkill` instead and also add it to the
non-optional list of binaries to install, which was revised also.
Fixes: https://github.com/dracutdevs/dracut/issues/1275