selinux: create standard user-runtime nodes with default context

Currently systemd-user-runtime-dir does not create the files in
/run/user/$UID/systemd/inaccessible with the default SELinux label.
The user and role part of these labels should be based on the user
related to $UID and not based on the process context of
systemd-user-runtime-dir.

Since v246-rc1 (9664be19) /run/user/$UID/systemd is also created by
systemd-user-runtime-dir and should also be created with the default
SELinux context.