Tree rules, current directories are removed. This is known by the master resolve file of a tree.

Tree name directories are removed. Services are not dependent anymore of a tree.
Tree name directories are replaced by a unique services directory containing all services of the system with it resolve file.

Tree resolve file handle now the list of the service name to start with a tree.

This change give the services switch between trees easily without IO access by the simplication
of the research, access of the resolve file and script of a service.

Try the current one, if unexisting take the default one.

The tree master resolve file handled this. No need to deal with IO.

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.

Thanks for your service! This resolve file was used with s6-rc. The things now are simplier
and flexible.