services: configuration: Change the value of the unset marker.

The new value of %unset-value sticks out more when something goes wrong, and
is also more unique; i.e. easier to search for.

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

gnu/services/configuration.scm

@@ -305,12 +305,13 @@ does not have a default value" field kind)))
 
 ;; Ideally this should be an implementation detail, but we export it
 ;; to provide a simpler API that enables unsetting a configuration
-;; field that has a maybe type, but also a default value.
+;; field that has a maybe type, but also a default value.  We give it
+;; a value that sticks out to the reader when something goes wrong.
 ;;
 ;; An example use-case would be something like a network application
 ;; that uses a default port, but the field can explicitly be unset to
 ;; request a random port at startup.
-(define %unset-value 'unset)
+(define %unset-value '%unset-marker%)
 
 (define (maybe-value-set? value)
   "Predicate to check whether a 'maybe' value was explicitly provided."

gnu/services/messaging.scm

@@ -95,7 +95,7 @@
                                  ;; doesn't interfere with
                                  ;; define-configuration and define-maybe
                                  ;; internals.
-                                 #''unset def))
+                                 #''%unset-marker% def))
                            #'(def ...) #'(target ...)))
                      ((new-doc ...)
                       (map (lambda (doc target)

tests/services/configuration.scm

@@ -150,7 +150,7 @@
   (protocol maybe-symbol ""))
 
 ;;; Maybe symbol values are currently seen as serializable, because the
-;;; unspecified value is 'unset, which is a symbol itself.
+;;; unspecified value is '%unset-marker%, which is a symbol itself.
 ;;; TODO: Remove expected fail marker after resolution.
 (test-expect-fail 1)
 (test-equal "symbol maybe value serialization, unspecified"