The StartTransientUnit call returns as soon as the job is enqueued, but
does not wait for systemd to actually do the work. Wait for the job
completion signal before continuing to exec, or until 1 second has
passed.
This avoids a race where a spawned child that quickly forks will have
only the parent process moved to the newly created cgroup, leaving the
early children in tmux's own cgroup. It also avoids problems if the
spawned process inspects or changes its own cgroup.
