Support a`--with-copy-program` compile flag

Postgres provides the `COPY .. TO/FROM PROGRAM` statement. This is dangerous from a security perspective because it allows users to escape from the SQL sandbox and gain shell access on the instance. 

Now there's the `pg_execute_server_program` predefined role to restrict access to `COPY.. TO/FROM PROGRAM` but if somehow a pg user gains superuser privileges then the predefined role is of no use.

So I wonder if we could remove the possibility of shell access by providing a `--with-copy-program` compile flag. 

Best regards,

Steve Chavez