system: Add (gnu system setuid).

I flubbed and somehow when including the patch series, this got dropped.
Oops... should fix things again.

* gnu/system/setuid.scm: New file.

gnu/system/setuid.scm

@@ -0,0 +1,57 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Brice Waegeneire <brice@waegenei.re>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu system setuid)
+  #:use-module (guix records)
+  #:export (setuid-program
+            setuid-program?
+            setuid-program-program
+            setuid-program-setuid?
+            setuid-program-setgid?
+            setuid-program-user
+            setuid-program-group
+
+            file-like->setuid-program))
+
+;;; Commentary:
+;;;
+;;; Data structures representing setuid/setgid programs.  This is meant to be
+;;; used both on the host side and at run time--e.g., in activation snippets.
+;;;
+;;; Code:
+
+(define-record-type* <setuid-program>
+  setuid-program make-setuid-program
+  setuid-program?
+  ;; Path to program to link with setuid permissions
+  (program       setuid-program-program) ;file-like
+  ;; Whether to set user setuid bit
+  (setuid?       setuid-program-setuid? ;boolean
+                 (default #t))
+  ;; Whether to set group setgid bit
+  (setgid?       setuid-program-setgid? ;boolean
+                 (default #f))
+  ;; The user this should be set to (defaults to root)
+  (user          setuid-program-user    ;integer or string
+                 (default 0))
+  ;; Group we want to set this to (defaults to root)
+  (group         setuid-program-group   ;integer or string
+                 (default 0)))
+
+(define (file-like->setuid-program program)
+  (setuid-program (program program)))