rrdimage: Switch graphs.pl to the new graph scripts

rrdimage: Switch graphs.pl to the new graph scripts

"makegraphbox" is modified to remove the old iframe method and output
a modern div container instead.
Graph errors are now returned, to be displayed by getrrdimage.cgi.

entropy.cgi and netovpnsrv.cgi are modified to ensure compatibility.

Add cache control HTTP header to image output.

Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>