Add an "adm" group

Journald will chown all journal files to the adm group so that users
in that group can run "journalctl".

modules/config/users-groups.nix

@@ -208,6 +208,7 @@ in
       users = { };
       nixbld = { };
       utmp = { };
+      adm = { }; # expected by journald
     };
 
     system.activationScripts.rootPasswd = stringAfter [ "etc" ]

modules/misc/ids.nix

@@ -123,6 +123,7 @@ in
     mpd = 50;
     clamav = 51;
     fprot = 52;
+    adm = 55;
 
     # When adding a gid, make sure it doesn't match an existing uid.