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.

Provide a tree by default and handle it name at compilation time with the --with-default-tree-name flag

rename tree_resolve_read_master_cdb and tree_resolve_write_master_cdb function to tree_resolve_master_read_cdb and tree_resolve_master_write_cdb respectively