services: urandom-seed: Deprecate the 'urandom-seed-service' procedure.

* gnu/services/base.scm (urandom-seed-service-type)[default-value]: New
field.
(urandom-seed-service): Mark as deprecated.
(%base-services): Use URANDOM-SEED-SERVICE-TYPE directly.
* gnu/services/base.scm (%base-services):
* doc/guix.texi (Base Services): Document 'urandom-seed-service-type'
instead of 'urandom-seed-service'.

doc/guix.texi

@@ -10031,12 +10031,12 @@ well as in the @var{groups} field of the @var{operating-system} record.
 @end example
 @end deffn
 
-@deffn {Scheme Procedure} urandom-seed-service
+@defvr {Scheme Variable} urandom-seed-service-type
 Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
 when rebooting.  It also tries to seed @file{/dev/urandom} from
 @file{/dev/hwrng} while booting, if @file{/dev/hwrng} exists and is
 readable.
-@end deffn
+@end defvr
 
 @defvr {Scheme Variable} %random-seed-file
 This is the name of the file where some random bytes are saved by

gnu/services/base.scm

@@ -597,12 +597,13 @@ (define urandom-seed-service-type
                        ;; start until we have seeded the PRNG.
                        (service-extension user-processes-service-type
                                           (const '(urandom-seed)))))
+                (default-value #f)
                 (description
                  "Seed the @file{/dev/urandom} pseudo-random number
 generator (RNG) with the value recorded when the system was last shut
 down.")))
 
-(define (urandom-seed-service)
+(define (urandom-seed-service)                    ;deprecated
   (service urandom-seed-service-type #f))
 
 
@@ -1984,7 +1985,7 @@ (define %base-services
                                           (ip "127.0.0.1")
                                           (provision '(loopback)))))
         (syslog-service)
-        (urandom-seed-service)
+        (service urandom-seed-service-type)
         (guix-service)
         (nscd-service)