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

Parse the service and write the result in a temporary directory. When the write process
is finished the directory is copied to the original service directory overwritten a
same file. The symlink /var/lib/66/system/.resolve/service/<name> is made at the complete
end of the process. So we can clean the mess removing the temporary directory and
avoid to change the original service directory before the end of the process. The state of
system should be clean event on crash of the parse process

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.