gnu: seatd-service-type: Use seat group.

* gnu/services/desktop.scm (seatd-group-sanitizer): New variable.
(<seatd-configuration>)[user]: Removed field.
[group]: Changed to "seat".  Sanitize via seatd-group-sanitizer.
(seatd-accounts): New variable.
(seatd-environment): Adjust to <seatd-configuration> ABI.
(seatd-service-type)[extensions]: Add account-service-type with seatd-accounts.
* gnu/tests/desktop.scm (run-minimal-desktop-test): Check for correct
ownership of $SEATD_SOCK.
* doc/guix.texi ("Desktop Services")[seatd-service-type]: Mention that users
may need to become members of the "seat" group.
Update default value for group field.
Add explanation on seatd.sock file.
Remove dropped user field.

gnu/tests/desktop.scm

@@ -255,6 +255,15 @@ minimal %BASE-SERVICES."
                    (socks (map wait-for-unix-socket-m socks)))
                 (and (= 2 (length socks)) (every identity socks)))))
 
+          (test-equal "seatd.sock ownership"
+            '("root" "seat")
+            `(,(marionette-eval
+                '(passwd:name (getpwuid (stat:uid (stat "/run/seatd.sock"))))
+                marionette)
+              ,(marionette-eval
+                '(group:name (getgrgid (stat:gid (stat "/run/seatd.sock"))))
+                marionette)))
+
           (test-assert "greetd is ready"
             (begin
               (marionette-type "ps -C greetd -o pid,args --no-headers > ps-greetd\n"