root: add an option to keep root read/write

In this case, the user has to provide a rootfs, otherwise, they may
damange their root filesystem. Also, it is not unlikely to display some
problems with 9p. No cleaning of /var/tmp, /tmp and /run is done in this
case either.
root: forbid the use of `--readwrite` without `--root`.

But a user can still force it by using `--root /`.