services: configuration: Support (field1 maybe-number "") format.

As opposed to explicitly using 'disabled as value, or using the
(field1 (maybe-number) "") format.

It's mostly the work of Maxime Devos shared under #54674, with some
modifications by Attila Lendvai.

* gnu/services/configuration.scm (normalize-field-type+def): New function.
(define-configuration-helper) (define-configuration): Support new field
format.
* tests/services/configuration.scm (config-with-maybe-number->string): New
function.
("maybe value serialization of the instance"): New test.
("maybe value serialization of the instance, unspecified"): New test.

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

tests/services/configuration.scm

@@ -27,6 +27,9 @@
 
 (test-begin "services-configuration")
 
+(define (serialize-number field value)
+  (format #f "~a=~a" field value))
+
 
 ;;;
 ;;; define-configuration macro.
@@ -47,7 +50,6 @@
   80
   (port-configuration-cs-port (port-configuration-cs)))
 
-(define serialize-number "")
 (define-configuration port-configuration-ndv
   (port (number) "The port number."))
 
@@ -101,15 +103,31 @@
 (define-maybe number)
 
 (define-configuration config-with-maybe-number
-  (port (maybe-number 80) "The port number."))
-
-(define (serialize-number field value)
-  (format #f "~a=~a" field value))
+  (port  (maybe-number 80) "")
+  (count maybe-number ""))
 
 (test-equal "maybe value serialization"
   "port=80"
   (serialize-maybe-number "port" 80))
 
+(define (config-with-maybe-number->string x)
+  (eval (gexp->approximate-sexp
+         (serialize-configuration x config-with-maybe-number-fields))
+        (current-module)))
+
+(test-equal "maybe value serialization of the instance"
+  "port=42count=43"
+  (config-with-maybe-number->string
+   (config-with-maybe-number
+    (port 42)
+    (count 43))))
+
+(test-equal "maybe value serialization of the instance, unspecified"
+  "port=42"
+  (config-with-maybe-number->string
+   (config-with-maybe-number
+    (port 42))))
+
 (define-maybe/no-serialization string)
 
 (define-configuration config-with-maybe-string/no-serialization