Set proper fallback CONFIG_DIR for NetBSD

This is not a critical fix since the official package for NetBSD uses
``--salt-config-dir`` to set the config dir. But, since NetBSD uses pkgsrc,
which defaults to /usr/pkg/etc, a NetBSD-specific fallback should be
set.

See https://github.com/saltstack/salt/issues/11889#issuecomment-40500747 for more information.