edit: Honor $VISUAL.

Suggested by Andreas Enge <andreas@enge.fr>.

* guix/scripts/edit.scm (%editor): Honor 'VISUAL' before 'EDITOR'.
  (show-help): Adjust accordingly.
* doc/guix.texi (Invoking guix edit): Likewise.

doc/guix.texi

@@ -3929,8 +3929,9 @@ guix edit gcc-4.8 vim
 @end example
 
 @noindent
-launches the program specified in the @code{EDITOR} environment variable
+launches the program specified in the @code{VISUAL} or in the
-to edit the recipe of GCC@tie{}4.8.4 and that of Vim.
+@code{EDITOR} environment variable to edit the recipe of GCC@tie{}4.8.4
+and that of Vim.
 
 If you are using Emacs, note that the Emacs user interface provides
 similar functionality in the ``package info'' and ``package list''

guix/scripts/edit.scm

@@ -38,7 +38,7 @@
 
 (define (show-help)
   (display (_ "Usage: guix edit PACKAGE...
-Start $EDITOR to edit the definitions of PACKAGE...\n"))
+Start $VISUAL or $EDITOR to edit the definitions of PACKAGE...\n"))
   (newline)
   (display (_ "
   -h, --help             display this help and exit"))
@@ -48,7 +48,8 @@ Start $EDITOR to edit the definitions of PACKAGE...\n"))
   (show-bug-report-information))
 
 (define %editor
-  (make-parameter (or (getenv "EDITOR") "emacsclient")))
+  (make-parameter (or (getenv "VISUAL") (getenv "EDITOR")
+                      "emacsclient")))
 
 (define (search-path* path file)
   "Like 'search-path' but exit if FILE is not found."