services: Use the new maybe/unset API.

* gnu/home/services/ssh.scm (serialize-address-family): Use the public API of
the maybe infrastructure.
* gnu/services/file-sharing.scm (serialize-maybe-string): Use maybe-value.
(serialize-maybe-file-object): Use maybe-value-set?.
* gnu/services/getmail.scm (getmail-retriever-configuration): Don't use
internals in unset field declarations.
(getmail-destination-configuration): Ditto.
* gnu/services/messaging.scm (raw-content?): Use maybe-value-set?.
(prosody-configuration): Use %unset-value.
* gnu/services/telephony.scm (jami-shepherd-services): Use maybe-value-set?.
(archive-name->username): Use maybe-value-set?.
* tests/services/configuration.scm ("maybe type, no default"): Use
%unset-value.

Modified-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

gnu/services/file-sharing.scm

@@ -114,10 +114,7 @@ type generated and used by Transmission clients, suitable for passing to the
 ;; name-value pair for the JSON builder.
 (set! serialize-maybe-string
   (lambda (field-name val)
-    (serialize-string field-name
-                      (if (eq? val 'unset)
-                          ""
-                          val))))
+    (serialize-string field-name (maybe-value val ""))))
 
 (define (string-list? val)
   (and (list? val)
@@ -180,9 +177,9 @@ type generated and used by Transmission clients, suitable for passing to the
 (define-maybe file-object)
 (set! serialize-maybe-file-object
   (lambda (field-name val)
-    (if (eq? val 'unset)
-        (serialize-string field-name "")
-        (serialize-file-object field-name val))))
+    (if (maybe-value-set? val)
+        (serialize-file-object field-name val)
+        (serialize-string field-name ""))))
 
 (define (file-object-list? val)
   (and (list? val)