sci-biology/foldingathome: systemd cleanup, various fixes.

sci-biology/foldingathome: systemd cleanup, various fixes.

1) force the `foldingathome` user group for the process for obvious
  reasons.
2) link in the video group and add it to the foldingathome user- this
  is necessary for GPU utilization.
3) force nice=19 from systemd level.  The batch processing attempts to
  do this for workers, but we might as well force it for the scheduler.
4) Add explicit configuration directives to force- irregardless of host config-
  the scheduler to be foreground'd and running in a fashion that integrates w/
  systemd more sanely.
5) Lockdown some basic systemd security while I'm in here.
6) Finally, drop the pidfile directive for systemd.  Foreground process-
  no pidfile to track (nor should we have the unit create it).

Bug: https://bugs.gentoo.org/712744
Signed-off-by: Brian Harring <ferringb@gmail.com>
Closes: https://github.com/gentoo/gentoo/pull/15145
Signed-off-by: Joonas Niilola <juippis@gentoo.org>