Let's make sure $XDG_RUNTIME_DIR for the user instance and /run for the
system instance is always organized the same way: the "inaccessible"
device nodes should be placed in a subdir of either called "systemd" and
a subdir of that called "inaccessible".
This way we can emphasize the common behaviour, and only differ where
really necessary.
Follow-up for #13823
To support ProtectHome=y in a user namespace (which mounts the inaccessible
nodes), the nodes need to be accessible by the user. Create these paths and
devices in the user runtime directory so they can be used later if needed.
This splits out a bunch of functions from fileio.c that have to do with
temporary files. Simply to make the header files a bit shorter, and to
group things more nicely.
No code changes, just some rearranging of source files.
Let's generalize this, so that we can use this in nspawn later on, which
is pretty useful as we need to be able to mask files from the inner
child of nspawn too, where the host's /run/systemd/inaccessible
directory is not visible anymore. Moreover, if nspawn can create these
nodes on its own before the payload this means the payload can run with
fewer privileges.
