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