r.mapcalc - Add warning when the same map is used both as an input and an output

[Hygsson]

A map cannot be used both as an input and as an output as in this invalid expression oldmap = oldmap + 1, instead a subsequent rename using g.rename is needed when the same name is desired:

r.mapcalc "newmap = oldmap + 1"
g.rename raster=newmap,oldmap

While (oldmap = oldmap + 1) may work on Linux, it will result in error on Windows.

As Wenzeslaus suggested in ticket:3857#comment:1, the solution might be to check inputs and outputs beforehand and add a warning for Linux and an error message for MS Windows.