systemd: replace use of `who` command
systemd: replace use of `who` command

Curiously the `who` command sometimes does not list logged-in users,
resulting in systemd not being reloaded. Instead we use

    systemctl --user is-system-running

to more directly detect whether systemd is running.

rycee/home-manager,rycee/home-manager,rycee/home-manager