core: only watch processes when it's really necessary

core: only watch processes when it's really necessary

If we know that main pid is our child then it's unnecessary to watch all
other processes of a unit since in this case we will get SIGCHLD when the main
process will exit and will act upon accordingly.

So let's watch all processes only if the main process is not our child since in
this case we need to detect when the cgroup will become empty in order to
figure out when the service becomes dead. This is only needed by cgroupv1.

Thanks Renaud Métrich for backporting this to RHEL.
Resolves: #1744972