system: Make PAM store SHA-512 encrypted passwords in /etc/shadow.

Fixes <http://bugs.gnu.org/21318>.

* gnu/system/linux.scm (unix-pam-service)[password]: Add 'sha512' and
  'shadow' to arguments.

gnu/system/linux.scm

@@ -148,7 +148,11 @@ (module "pam_unix.so"))))
                           (module "pam_unix.so")
                           (arguments '("nullok")))
                          unix)))
-         (password (list unix))
+         (password (list (pam-entry
+                          (control "required")
+                          (module "pam_unix.so")
+                          ;; Store SHA-512 encrypted passwords in /etc/shadow.
+                          (arguments '("sha512" "shadow")))))
          (session (if motd
                       (list unix
                             (pam-entry