v.to.db: Add --overwrite to help and GUI (#807)

Address #794.

vector/v.to.db/main.c

@@ -52,6 +52,7 @@ int main(int argc, char *argv[])
     G_add_keyword(_("sinuous"));
     G_add_keyword(_("slope"));
     module->description = _("Populates attribute values from vector features.");
+    module->overwrite = 1;
 
     parse_command_line(argc, argv);