[tlbuild] mpfi integration into (u)(p)mpost

[Karl Berry]

Hi TL builders - Luigi has integrated the mpfi library for interval
arithmetic into mpost (and consequently also into (u)pmpost).  He made
the new version of base mpost be 2.10.

I committed the changes just now in r69664. We'll see what the
autobuilds make of it; those of you who build manually might want to
give it a try to avoid unpleasant surprises later. Hope it doesn't cause
any trouble. Fingers crossed ... --thanks, karl.