[GRASS5] new raster fixes

[Markus Neteler]

Hi,

I have fixed a few things:
 - r.thin writes NULL instead of 0 (src/raster/r.thin/cmd/io.c)
 - r.digit writes NULL instead of zero
   (fix applied in src/raster/r.in.poly/raster.c)
   For the latter I am not sure how we treat 0 categories.
   But the NULL fix was needed to avoid other problems.

 - r.compress, r.null, r.clump, r.reclass: improved warning when
   a reclassed map is found (tell user the name/mapset of the related
   base map now)
 - r.neighbors added test for reclassed maps since it obviously doesn't
   work with those

Good night,

 Markus