Use domain_search_flags in lookup_symbol et al

Use domain_search_flags in lookup_symbol et al

This changes lookup_symbol and associated APIs to accept
domain_search_flags rather than a domain_enum.

Note that this introduces some new constants to Python and Guile.  I
chose to break out the documentation patch for this, because the
internals here do not change until a later patch, and it seemed
simpler to patch the docs just once, rather than twice.