lx_fs: evaluate session argument for client-selected directory

[nfeske]

While working on #5445, I noticed that lx_fs does not append the client-selected (sub-)directory to the root dir attribute value of the <policy> node. This is needed to combine lx_fs with the chroot component or with the upcoming label-rewriting-based chrooting. Apparently, the existing use cases of lx_fs have not called for this as lx_fs is easy enough to be instantiated per directory. But as the behavior is surprising and inconsistent with the vfs server, we should better implement it.