Add and integrate sdn-open

Originally I thought that not supporting %cd would be an issue,
making this kind of utility unclean.

It turns out the desire to launch xdg-open quickly is stronger.

NEWS

@@ -5,7 +5,7 @@ Unreleased
     - + and - adjust the selection using shell globs;
     - t and T insert the selection into the external command line
       in relative or absolute form, respectively;
-	- Enter is like t but enters directories, and M-Enter is synonymous to t;
+    - Enter is like t but enters directories;
     - C-g or Escape clear the selection, similarly to the editor.
 
  * Added an sdn-view script that can process Midnight Commander mc.ext.ini files
@@ -13,6 +13,9 @@ Unreleased
    while the original direct pager invocation has been moved to F13 (which also
    reflects Midnight Commander)
 
+ * Added an sdn-open script which does the same kind of processing as above
+   on top of xdg-open.  This is what is now executed by M-Enter.
+
 
 1.0.0 (2024-12-21)