[GRASS5] an idea re. "d.zoom finished." etc.

Hi,

Perhaps we should make better use of G_done_msg("").

..what if we had the parser set a GRASS_IS_GUI enviro var right before
the fork()? then modules that wanted to do something different in GUI
mode (such as display a "Finished" message) could do so with a bit more
control.

This doesn't reach the goal of automatically adding a "finished" message
to all modules in GUI mode, but maybe a start?

Hamish