pg_basebackup: Fix a couple of recently-introduced bugs.

pg_basebackup: Fix a couple of recently-introduced bugs.

The server expects the compression level to be between 1 and 9, but
Z_DEFAULT_COMPRESSION is -1, so we must not try to send that value
to the server.

Because pg_basebackup's -R option is implemented on the client side,
it can't be used in combination with a backup target. Error out if
someone tries that, instead of silently ignoring the option.

Both issues were reported by Tushar Ahuja; patch by me.

Discussion: http://postgr.es/m/CA+TgmoaMwgdx8HxBjF8hmbohVvPL_0H5LqNrSq0uU+7BKp_Q2A@mail.gmail.com