system: Change the shell of 'nobody' to 'nologin'.

Fixes <http://bugs.gnu.org/23971>.
Reported by Vincent Legoll <vincent.legoll@gmail.com>.

* gnu/system/shadow.scm (%base-user-accounts): Add 'shell' field.  Set
'home-directory' to "/nonexistent".

gnu/system/shadow.scm

@@ -126,7 +126,8 @@ (define %base-user-accounts
          (name "nobody")
          (uid 65534)
          (group "nogroup")
-         (home-directory "/var/empty")
+         (shell #~(string-append #$shadow "/sbin/nologin"))
+         (home-directory "/nonexistent")
          (system? #t))))
 
 (define (default-skeletons)