[exim] Eximstats average message size

Gregor,

Thanks for your patch to display the average message sizes in eximstats.

There were a couple of bugs in it, as it only worked for message size
totals under 1 GB, and broke merging of old reports.

I've incorporated your patch and fixed these issues in eximstats-1.36,
which can be downloaded from www.computurn.com/exim.

Steve Campbell