Use systemd consistently if availabile for p4d start/stop.
The load_checkpoint.sh has logic to start using systemd if
available, else use the underlying init script directly. We
need to get that logic to be used anywhere that p4d is started
That includes (at least):
* Somewhere in backup_functions.sh (in the routine called when
swapping dbs when doing checkpoint regeneration, which, while
not called by current scripts, should come back some day).