With the updated bufnorm code, buffered 'z drivers are used as anchor
points for undirected connections. These are currently not supported by
read/write_xaiger2, so we temporarily replace those by roughly
equivalent $tribuf cells which will be handled as blackboxes that
properly roundtrip through the xaiger2 front and backend.
Previously the `abc9_box` mode was reserved to modules with the
`blackbox` or `whitebox` attribute. Allow `abc9_box` on ordinary modules
when doing hierarchical synthesis.
