Make USERs a special case of ROLE

I am trying to have one user masquerade as another: grants, default owner/definer, etc.

This is trivial to do in PostgreSQL because that DB treats “users” merely as “roles that can log in”.

MariaDB’s implementation of ROLEs doesn’t allow this, though it seems a very useful feature. It’s kinda-sorta possible via MariaDB’s PAM auth plugin and perhaps a custom PAM plugin that would rewrite the username, but being able to do this with MariaDB’s native authentication would be most ideal.