system: pam: Let PAM extensions add shepherd requirements.

* gnu/system/pam.scm (<pam-extension>): New record type.
(pam-shepherd-service): Add Shepherd synchronization point.

* gnu/services/mail.scm (dovecot-shepherd-service)
* gnu/services/lightdm.scm (lightdm-shepherd-service)
* gnu/services/mail.scm (opensmtpd-shepherd-service)
* gnu/services/sddm.scm (sddm-shepherd-service)
* gnu/services/ssh.scm (lsh-shepherd-service, openssh-shepherd-service)
* gnu/services/xorg.scm (slim-shepherd-service, gdm-shepherd-service)
* gnu/services/base.scm (greetd-shepherd-services): Add PAM requirement.

* gnu/system/pam.scm (/etc-entry, extend-configuration,
pam-root-service-type, pam-root-service)
* gnu/services/authentication.scm (pam-ldap-pam-service)
* gnu/services/base.scm (pam-limits-service-type)
(greetd-pam-service)
* gnu/services/desktop.scm (pam-gnome-keyring)
* gnu/services/kerberos.scm (pam-krb5-pam-service)
* gnu/services/pam-mount.scm (pam-mount-pam-service): Adapt to use
pam-extension.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/xorg.scm

@@ -667,7 +667,7 @@ reboot_cmd " shepherd "/sbin/reboot\n"
 
                        (list (symbol-append 'xorg-server-
                                             (string->symbol vt)))))
-           (requirement '(user-processes host-name udev))
+           (requirement '(pam user-processes host-name udev))
            (start
             #~(lambda ()
                 ;; A stale lock file can prevent SLiM from starting, so remove it to
@@ -1119,7 +1119,7 @@ argument.")))
   (list (shepherd-service
          (documentation "Xorg display server (GDM)")
          (provision '(xorg-server))
-         (requirement '(dbus-system user-processes host-name udev elogind))
+         (requirement '(dbus-system pam user-processes host-name udev elogind))
          (start #~(lambda ()
                     (fork+exec-command
                      (list #$(file-append (gdm-configuration-gdm config)