Add safety check to ensure systemd is used if required.
We want a reliable way to prevent the SysV init scripts in
the SDP from being called directly (i.e. not via systemd)
on systems where systemd is required. This includes
* RHEL/CentOS 7+,
* Ubuntu 16.04+,
* the latest Amazon Linux
* etc.
This works around a known issue with systemd: If the old SysV
init scripts are called directly, rather than via the systemd
mechanism, then they won't be called to shutdown gracefully on
reboot. Thus a reboot could lead to a hard kill of p4d rather
than a graceful shutdown, potentially leading to database and/or
journal corruption.