Fix screensaver::disable systemd service status.

Turns out that the systemd doesn't consider the 'screensaver-disabled'
service as running after the setterm command has exited, as we had it
configured.  We mistakenly thought it would if the service Type was
"oneshot", but it doesn't, so 'systemctl status screensaver-disabled'
would return 3 except during the milliseconds while setterm(1) was
actually running.  This causes Puppet to start it again and again
every time Puppet runs.

The solution to this is to set the 'RemainAfterExit' option to "yes"
in the service unit file.  Then systemd will consider the service to
be running even if the process has exited (as long as it has run at
least once).  Do so.