Make use of the new immutable-flag ioctl helpers. While at it, move the
`chmod` to `fchmod` and re-use the open file-descriptor. Document the
behavior and move the `fchmod` into its own try-block for the same
reasons as the `ioctl` call: We rely on the following unlink() to catch
any errors. Errors in the fixperms() step are non-consequential.
Move remove_tree() into its own module in `osbuild.util.rmrf`. This way
we can use it in other modules as well, without cross-referencing
internal helpers.