shmem_exit to replace exitpg().

From: t-ishii@sra.co.jp

As mentioned around line 1153 in backend/commands/copy.c, the method
of array checking is not perfect.

test=> create table t1 (i text);
test=> insert into t1 values('{\\.}');
INSERT 2645600 1
test=> select * from t1;
i
-----
{\\.}
(2 rows)
test=> copy t1 to '/tmp/aaa';
test=> copy t1 from '/tmp/aaa';
ERROR:  CopyReadAttribute - end of record marker corrupted

Copy cannot read data produced by itself!

real small function to revoke update on a column. The function >
> doesn't do anything > > fancy like checking user ids.  > > > >
I copied most of it from the refint.c in the contrib directory.
> > > > Should I post this somewhere? It really isn't very big.
> >

Here it is...

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