Everything of use to frontend code should now appear in the _d.h files,
and making this change frees us from needing to worry about whether the
catalog header files proper are frontend-safe.

Remove src/interfaces/ecpg/ecpglib/pg_type.h entirely, as the previous
commit reduced it to a confusingly-named wrapper around pg_type_d.h.

In passing, make test_rls_hooks.c follow project convention of including
our own files with #include "" not <>.

Discussion: https://postgr.es/m/23690.1523031777@sss.pgh.pa.us