system: Add privileged-programs to <operating-system>.

* gnu/system.scm (<operating-system>): Add new privileged-programs
field, that defaults to…
(%default-privileged-programs): …this new variable, renamed from…
(%setuid-programs): …this, which is now defined as the empty list.
* doc/guix.texi (Setuid Programs): Rename this…
(Privileged Programs): …to this.  Adjust all refs.  Update all mentions
of ‘setuid’ (whether in prose, variable names, or code samples) to use
the new ‘privilege[d]’ terminology instead.
(operating-system Reference, X Window, Invoking guix system)
(Service Reference): Adjust likewise.

gnu/services.scm

@@ -47,7 +47,6 @@
   #:use-module (gnu packages bash)
   #:use-module (gnu packages hurd)
   #:use-module (gnu system privilege)
-  #:use-module (gnu system setuid)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)