rfe: r.md5sum or equivalent

[hamish]

Hi,

It would be helpful for comparing maps if there was some r.md5sum or equivalent.

Actually I think a new flag for r.univar might be all that is needed.

presently this is what we got, and it's a bit less than optimal, especially for quickly running through a test suite

r.mapcalc diff=map1-map2
r.univar diff
g.remove diff

thanks, Hamish