Allow internal-only location resources
Location resources with internal == true are valid, 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.

This is a re-implementation of jfryman/nginx PR-388.
Merge pull request #464 from danieldreier/allow_internal_only

Allow internal-only location resources