Allow CHECK constraints to be declared ONLY

This makes them enforceable only on the parent table, not on children
tables.  This is useful in various situations, per discussion involving
people bitten by the restrictive behavior introduced in 8.4.

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CAFaPBrSMMpubkGf4zcRL_YL-AERUbYF_-ZNNYfb3CVwwEqc9TQ@mail.gmail.com

Authors: Nikhil Sontakke, Alex Hunsaker
Reviewed by Robert Haas and myself