sysext: Refactor the merge hierarchy code

sysext: Refactor the merge hierarchy code

Divide the merge_hierarchy function into code that:

- determines the lower directories for overlayfs

  - determination of lower directories was further split into top, middle and
    bottom directories:

    - bottom - possibly the hierarchy itself

    - middle - hierarchies from extensions

    - top - metadata directory

- mounts the overlayfs using determined directories

- writes information to the metadata directory

- makes the merged hierarchy read-only