services: rottlog: Make extensible.

* gnu/services/admin.scm (rottlog-service-type)[compose, extend]: New
fields.
* doc/guix.texi (Log Rotation): Mention extension.

doc/guix.texi

@@ -9561,6 +9561,9 @@ with the default settings, for commonly encountered log files.
 This is the type of the Rottlog service, whose value is a
 @code{rottlog-configuration} object.
 
+Other services can extend this one with new @code{log-rotation} objects
+(see below), thereby augmenting the set of files to be rotated.
+
 This service type can define mcron jobs (@pxref{Scheduled Job
 Execution}) to run the rottlog service.
 @end defvr

gnu/services/admin.scm

@@ -164,6 +164,12 @@ for ROTATION."
                      ;; the documentation.
                      (service-extension profile-service-type
                                         (compose list rottlog-rottlog))))
+   (compose concatenate)
+   (extend (lambda (config rotations)
+             (rottlog-configuration
+              (inherit config)
+              (rotations (append (rottlog-rotations config)
+                                 rotations)))))
    (default-value (rottlog-configuration))))
 
 ;;; admin.scm ends here