add servicedir entry at live addon. Add enabled at configuration group entry. Move status entry from path to live addon

simplify interface of service_is_g and its functionality, provide service_get_treename as replacement

This commit sign the end of the "[WIP] allow to switch between trees" works.
The tree directory was used with s6-rc. Tree are now completely handled by
resolve file. That's means that services are not independent of a tree.
Again earn of flexibility.