services: configuration: Use *unspecified* instead of 'disabled.

Use *unspecified* as a marker for field values that have not been set.

Rationale: 'disabled may easily clash with user values for boolean fields, is
confusing (i.e. its meaning is *not* boolean false, but unspecified) and it
also passes silently through the symbol? predicate of a field of type symbol.

* gnu/services/configuration.scm (configuration-missing-default-value):
Renamed from configuration-no-default-value.
(define-maybe-helper): Use *unspecified* instead of 'disabled, and make
the default value optional.
* gnu/home/services/desktop.scm (home-redshift-configuration):
Change (maybe-xyz 'disabled) to maybe-xyz.
* gnu/services/authentication.scm (nslcd-configuration): Likewise.
* gnu/services/cgit.scm (repository-cgit-configuration): Likewise.
* gnu/services/file-sharing.scm (serialize-maybe-string)
(serialize-maybe-file-object): Use 'unspecified?' instead of (eq? val
'disabled).
* gnu/services/messaging.scm (raw-content?): Likewise.
(ssl-configuration): Change (maybe-xyz 'disabled) to maybe-xyz.
(prosody-configuration): Likewise.
* gnu/services/file-sharing.scm (transmission-daemon-configuration):
Likewise.
* gnu/services/messaging.scm (define-all-configurations):
Use *unspecified* instead of 'disabled'.
* gnu/services/networking.scm (opendht-configuration): Likewise.
* gnu/services/pm.scm (tlp-configuration): Likewise.
* gnu/services/telephony.scm (jami-account): Likewise.
(jami-configuration): Likewise.
* gnu/services/vpn.scm (openvpn-client-configuration): Likewise.
* tests/services/configuration.scm ("maybe type, no default")
("maybe type, with default"): New tests.

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

gnu/services/cgit.scm

@@ -241,27 +241,27 @@ is no suitable HEAD.")
    (repo-file-object "")
    "Override the default @code{email-filter}.")
   (enable-commit-graph?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-commit-graph?}.")
   (enable-log-filecount?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-log-filecount?}.")
   (enable-log-linecount?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to disable the global setting
 @code{enable-log-linecount?}.")
   (enable-remote-branches?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "Flag which, when set to @code{#t}, will make cgit display remote
 branches in the summary and refs views.")
   (enable-subject-links?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to override the global setting
 @code{enable-subject-links?}.")
   (enable-html-serving?
-   (maybe-repo-boolean 'disabled)
+   maybe-repo-boolean
    "A flag which can be used to override the global setting
 @code{enable-html-serving?}.")
   (hide?