services: configuration: Step back from *unspecified*.

Fixes <https://issues.guix.gnu.org/56799>.

This partially reverts 8cb1a49a3998c39f315a4199b7d4a121a6d66449.

Rationale: *unspecified* cannot be serialized thus used as a G-Expression
input, which is problematic/inconvenient when using deeply nested records.  As
an example, jami-service-type was broken when using partially defined
<jami-account> records.

* gnu/services/configuration.scm (define-maybe-helper): Check against the
'unset symbol.
(normalize-field-type+def): Adjust value to 'unset.
(define-configuration-helper): Use 'unset as the default value thunk.
* gnu/services/file-sharing.scm (serialize-maybe-string): Check against the
'unset symbol.
(serialize-maybe-file-object): Likewise.
* gnu/services/messaging.scm (define-all-configurations): Use 'unset as
value.
(raw-content?): Check against 'unset symbol.
(prosody-configuration)[http-max-content-size]: Default to 'unset.
[http-external-url]: Likewise.
[mod-muc]: Likewise.
[raw-content]: Likewise.
* gnu/services/networking.scm (opendht-configuration): Adjust documentation.
* gnu/services/telephony.scm (jami-shepherd-services): Replace *undefined*
with the 'unset symbol.
* tests/services/configuration.scm ("maybe type, no default"): Check against
the 'unset symbol.
* doc/guix.texi: Regenerate the opendht-configuration,
openvpn-client-configuration and openvpn-server-configuration documentation.

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