pciguard: Add user and group

Add new user and group for the external PCI authorization daemon.

BUG=b:171541662,b:167448768
TEST=display-accts.py --lint

Change-Id: I302443accc954028437c47abdb62928e6fbc3078
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2543285
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Aashay Shringarpure <aashay@google.com>
Tested-by: Rajat Jain <rajatja@google.com>
Commit-Queue: Rajat Jain <rajatja@google.com>