Ensure that updating the file preserves its mode

Preserve the file mode when replacing it, this notably keeps
docs/doxygen/regen.sh executable when running misc/scripts/inc_year.

This uses GNU chmod --reference option which is not available on all
systems, but it shouldn't be a problem in practice.

misc/scripts/run_sed_common.sh

@@ -37,6 +37,7 @@ run_sed() {
         rm $filename.$$
         msgc "unchanged"
     else
+        chmod --reference=$filename $filename.$$
         mv $filename.$$ $filename
         msgc "done"
     fi