Fixed so the p4dtg_N (P4DTG) service can be managed with systmectl, e.g.
sudo systemctl start p4dtg_1
sudo systemctl status p4dtg_1
sudo systemctl stop p4dtg_1
Also added support for a "force_start" option, similar to that being added for
p4d (per SDP-837).
This change includes:
* Modifiedcation to the temmplate for the p4dtg_N_init script.
* Addition of new template for the systemd p4dtg_N.service file.
* Added a new script to generate a systemctl-compatible environment file from
the standard SDP shell environment mechanism.