unlink: preserve non-keg-only versioned links

Adjust unlink_link_overwrite_formulae so non-keg-only formulae only unlink
linked keg-only overwrite siblings before relinking.

This avoids removing coexistable versioned links like python3.13 and
python3.14 when linking non-keg-only formula families, while keeping the
existing full-unlink behavior for keg-only formulae.

Update unlink_spec to cover both behaviors:
- non-keg-only formulae only unlink linked keg-only siblings
- keg-only formulae still unlink all linked overwrite siblings