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

Hello hackers,

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