[PATCH] Better logging of COPY queries if log_statement='all'

Hello.

Sometimes it's useful to log content of files used in COPY ... TO ... and
COPY ... FROM ... queries. Unfortunately PostgreSQL doesn't allow to do
it, even if log_statement='all'. Suggested patch fixes this.

Log example:

```
LOG: statement: create table test (k int, v text);
LOG: statement: insert into test values (111, 'aaa'), (222, 'bbb');
LOG: statement: copy test to '/tmp/copy.txt';
LOG: statement: 111 aaa
LOG: statement: 222 bbb
LOG: statement: delete from test;
LOG: statement: copy test from '/tmp/copy.txt';
LOG: statement: 111 aaa
LOG: statement: 222 bbb
```
--
Best regards,
Aleksander Alekseev