State on each SQL signature that it returns a set

[estebanzimanyi]

A signature declared RETURNS SETOF carries retSet, PostgreSQL's proretset,
beside ret, the type of one of its rows: unnest(intset) is
{args: [intset], ret: integer, retSet: true, sqlName: unnest}, while
getValues(intset), over the same intset_values, returns one integer[] and
carries no retSet. The RETURNS match of _create_fn_stmts captures SETOF
instead of dropping it, the signatures of one function are told apart by it,
and a bound literal is matched to its signature with it.

Why. Flink and Spark carry a set-returning signature as one function
returning an array that the query unfolds into rows, and a signature
returning one value as a scalar function. The row type alone does not tell
the two apart, and the C shape does not either: an array return backs
getValues as much as unnest.

Measured. Over MobilityDB a362004728, 136 SQL signatures carried by 80
functions state retSet: unnest, the splits, the tiles, the …Pairs kernels,
frechetDistancePath, dynTimeWarpPath and dumpAsPolygons. The catalog is
otherwise identical to the one derived without the change.

Witness. tests/test_sqlfn_setof.py reads SETOF in either case, keeps it
apart from the row type, and states it on the unnest signature of
intset_values and not on its getValues signature.