Throw a "function protocol violation" error if a function in FROM
tries to return a set though it wasn't marked proretset.  Although
such cases work at the moment, it doesn't seem like something we
want to guarantee will keep working.  Besides, there are other
negative consequences of not setting the proretset flag, such as
potentially bad plans.

No back-patch, since if there is any third-party code violating
this expectation, people wouldn't appreciate us breaking it in
a minor release.

Discussion: https://postgr.es/m/1636062.1615141782@sss.pgh.pa.us