filename suggestion when downloading graphs

[chird]

Hi,

I propose a change in graph_image.php to suggest the browser with a filename to save the image as, if a user wants to save a graph.
I use this all the time when I want to save and send graphs to 3rd persons.

Proposal, after line 35:
header("Content-disposition: inline; filename=cacti-".date("Ymd_His",time()).".png");

Any other/better/more dynamic default filenames are welcome of course.

(Excuse me for not sending a proper patch...)

Regards,
Geert