[GRASS-dev] [bug #4368] (grass) g.parser: pass through --o

[Cedric Shock via RT]

parser.c now sets the OVERWRITE environment variable, but doesn't export it.
Scripts run by g.parser now have it set so that modules they run will be run
as if --o was included. It can be used here to skip the overwrite check that's
implemented in r.blend to check for .r .g and .b extensions to the map name.

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