[GRASS5] d.legend improvement

[Markus Neteler]

Hi,

to improve d.legend a bit, I have added a new flag: -o

-o Draw entries only for existing categories

The flag -o is useful for categorial maps, it suppresses the
drawing of non-existing categories (otherwise the full range is shown).
This flag is ignored for FP maps.

Idea: Now you can get a legend such as:

100 sea
200 land
9000 road

Without -o it is
100 sea
101
102
[...]
199
200 land
201
[...]

say, a bit ugly for this categorial map type.

Cheers,

 Markus