Do not warn the user about this feature. It only used on very specific case like module migration process.
Badly used, this features can easily broke the sanity of the system state

add service_resolve_write_tmp and state_write_tmp function to be able to write those files at a temporary directory

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.