hidepid: polkit and systemd-logind compatibility

`systemd.hideProcessInformation = true`, would break interactions
requiring polkit arbitration such as initating poweroff/reboot as a
normal user; the polkit daemon cannot be expected to make decisions
about processes that don't exist as far as it is concerned.

systemd-logind lacks the `sys_ptrace` capability and so needs to be part
of the designated proc gid, even though it runs as root.

Fixes https://github.com/NixOS/nixpkgs/issues/20948

nixos/modules/security/hidepid.nix

···
 
   config = mkIf config.security.hideProcessInformation {
     users.groups.proc.gid = config.ids.gids.proc;
+    users.groups.proc.members = [ "polkituser" ];
 
     boot.specialFileSystems."/proc".options = [ "hidepid=2" "gid=${toString config.ids.gids.proc}" ];
+    systemd.services.systemd-logind.serviceConfig.SupplementaryGroups = [ "proc" ];
   };
 }