tmpfiles: don't do automatic cleanup in $XDG_RUNTIME_DIR

Now that logind will clean up all IPC resources of a user we should
really consider $XDG_RUNTIME_DIR as just another kind of IPC with the
same life-cycle logic as the other IPC resources. This should be safe
now to do since every user gets his own $XDG_RUNTIME_DIR tmpfs instance
with a fixed size limit, so that flooding of it will more effectively be
averted.

tmpfiles.d/systemd.conf

@@ -7,7 +7,7 @@
 
 # See tmpfiles.d(5) for details
 
-d /run/user 0755 root root ~10d
+d /run/user 0755 root root -
 F! /run/utmp 0664 root utmp -
 
 d /run/systemd/ask-password 0755 root root -