ui: 'package->recutils' accurately honors the number of columns.

* guix/ui.scm (package->recutils)[width*]: New variable.  Use it instead
of WIDTH.

guix/ui.scm

@@ -855,11 +855,16 @@ followed by \"+ \", which makes for a valid multi-line field value in the
 (define* (package->recutils p port #:optional (width (%text-width)))
   "Write to PORT a `recutils' record of package P, arranging to fit within
 WIDTH columns."
+  (define width*
+    ;; The available number of columns once we've taken into account space for
+    ;; the initial "+ " prefix.
+    (if (> width 2) (- width 2) width))
+
   (define (dependencies->recutils packages)
     (let ((list (string-join (map package-full-name
                                   (sort packages package<?)) " ")))
       (string->recutils
-       (fill-paragraph list width
+       (fill-paragraph list width*
                        (string-length "dependencies: ")))))
 
   (define (package<? p1 p2)
@@ -901,7 +906,7 @@ WIDTH columns."
   (format port "~a~2%"
           (string->recutils
            (string-trim-right
-            (parameterize ((%text-width width))
+            (parameterize ((%text-width width*))
               (texi->plain-text
                (string-append "description: "
                               (or (and=> (package-description p) P_)