nixos: add optional process information hiding

This module adds an option `security.hideProcessInformation` that, when
enabled, restricts access to process information such as command-line
arguments to the process owner.  The module adds a static group "proc"
whose members are exempt from process information hiding.

Ideally, this feature would be implemented by simply adding the
appropriate mount options to `fileSystems."/proc".fsOptions`, but this
was found to not work in vmtests. To ensure that process information
hiding is enforced, we use a systemd service unit that remounts `/proc`
after `systemd-remount-fs.service` has completed.

To verify the correctness of the feature, simple tests were added to
nixos/tests/misc: the test ensures that unprivileged users cannot see
process information owned by another user, while members of "proc" CAN.

Thanks to @abbradar for feedback and suggestions.

nixos/modules/misc/ids.nix

@@ -47,6 +47,7 @@
       #floppy = 18; # unused
       #uucp = 19; # unused
       #lp = 20; # unused
+      #proc = 21; # unused
       pulseaudio = 22; # must match `pulseaudio' GID
       gpsd = 23;
       #cdrom = 24; # unused
@@ -288,6 +289,7 @@
       floppy = 18;
       uucp = 19;
       lp = 20;
+      proc = 21;
       pulseaudio = 22; # must match `pulseaudio' UID
       gpsd = 23;
       cdrom = 24;