I've noticed that several methods of opening a "login shell" do not
result in the XDG_RUNTIME_DIR (/run/user/<uid>) being created; and also
don't result in a session appearing in the output of `loginctl`:


* `mosh`
* `sudo -i <user> loginctl`
* `su -l <user>`
* `login`

I mentioned in another message that I'm hoping to write a system
shepherd service that will start a user-level shepherd service. But a
user-level shepherd services won't run without the XDG_RUNTIME_DIR (or
some other explicitly-chosen suitable directory, but I'd prefer not to
deviate from the defaults, if I could instead understand what's going on).

Does anyone know why this would happen, or how to fix it? I'm using the
elogind service on top of %base-services.