Alix 2D6 crashes upgrade process withou out of diskspace

Updating the the RRD graphs causes two copies of each RRD's XML file to be stored in /tmp.

On Nanobsd, the default /tmp size is 40mb. It doesn't require very many RRD XML dumps before this is exhausted.
Merge pull request #802 from Xon/patch-1

Alix 2D6 crashes during 2.1 upgrade process with an out of diskspace error