Fix RETURNING side effects for FOR PORTION OF leftover rows

Hi,

Dean raised a comment on my patch [1] about RETURNING processing for UPDATE ... FOR PORTION OF. While addressing that comment, I found a new problem: UPDATE/DELETE ... FOR PORTION OF currently processes RETURNING for leftover rows as well, which seems wrong because it can cause unexpected side effects.

Here is a simple repro using a sequence:
```
evantest=# create sequence s;
CREATE SEQUENCE
evantest=# create table t (r int4range, val int);
CREATE TABLE
evantest=# insert into t values ('[1,4)', 0);
INSERT 0 1
evantest=# update t for portion of r from 2 to 3 set val=1 returning nextval('s') as n;
n
---
3
(1 row)

UPDATE 1
evantest=# select last_value from s;
last_value
------------
3
(1 row)
```

The UPDATE FOR PORTION OF command generates two leftover rows, and both cause RETURNING to be processed. As a result, the sequence is advanced three times, which seems unexpected from the user's perspective.

To fix the problem, since leftover rows are inserted by calling ExecInsert(), and ExecInsert() is a local static function, I think a simple and safe solution is to add a new parameter to ExecInsert() to indicate whether to process RETURNING for this insert. Then UPDATE/DELETE FOR PORTION OF can bypass RETURNING processing for leftover rows.

See the attached patch for details.

[1] https://postgr.es/m/6C34A987-AC50-4477-BD71-2D4AFEE1A589@gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/