Allow internal-only location resources

This is a re-implementation of #388 because it hasn't been rebased and I need the changes it makes.

Location resources with internal == true are valid in nginx, but the module does
not allow them unless one or more of www_root, proxy, location_alias,
stub_status, fastcgi, or location_custom_cfg are set

This patch allows locations with none of those but internal == true to
be created.