Reject non-ON-SELECT rules that are named "_RETURN".

Reject non-ON-SELECT rules that are named "_RETURN".

DefineQueryRewrite() has long required that ON SELECT rules be named
"_RETURN".  But we overlooked the converse case: we should forbid
non-ON-SELECT rules that are named "_RETURN".  In particular this
prevents using CREATE OR REPLACE RULE to overwrite a view's _RETURN
rule with some other kind of rule, thereby breaking the view.

Per bug #17646 from Kui Liu.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/17646-70c93cfa40365776@postgresql.org