RI fastpath misses checking EXECUTE on functions

Hi,

It was pointed out to me offlist by Nikolay Samokhvalov that the RI
fast path (ri_FastPathCheck) doesn't check EXECUTE privilege on the
functions it invokes on the FK values. There are two such functions:
the equality operator's function, which goes into the ScanKey the
index AM evaluates, and the implicit cast function, which is used when
the FK value must be converted to the PK column's type. In the SPI
path's case, because the operator appears in the generated query's
WHERE clause and the cast as $1::pktype, ExecutorStart checked EXECUTE
on both as part of initializing the query, as the referenced table's
owner. So while I added ri_CheckPermissions() to parallel the
executor's ExecCheckPermissions(), I missed the non-relation
permission checks that occur as part of expression initialization.

The attached patch adds ri_CheckFunctionPermissions(), which does
object_aclcheck(ProcedureRelationId, ..., ACL_EXECUTE) plus
InvokeFunctionExecuteHook() for each of the two functions, and calls
it once the fast-path metadata is populated. That puts the checks at
the same point as the existing relation permission check.

Regression tests are included for both functions. They reach the fast
path through per-row validation of a new constraint, and repeat each
case against a partitioned referenced table, which takes the SPI path,
to establish that the two behave identically.

I'll add an open item. I would like to get this committed by EOW if
there are no comments.

--
Thanks, Amit Langote