Add confirmation prompt before dropping existing xref database.

misc-scripts/xref_mapping/xref_parser.pl

@@ -64,7 +64,8 @@ sub usage {
                 used.
 
   -create       If specified, cause dbname to be deleted and re-created if it
-                already exists.
+                already exists. User is prompted before database is dropped to
+                prevent disasters arising from dropping the wronf database.
 
   -skipdownload Don't download new data, parse existing.