allow ranges to be disabled * ov.cc, ov.h: Allow creation of range object to be disabled. Also allow range objects to be forced, even when generally disabled. * pt-exp.h (tree_expression::for_cmd_expr): New member variable. (tree_expression::mark_as_for_cmd_expr, tree_expression::is_for_cmd_expr): New functions. * oct-parse.in.yy: Mark for command expressions. * pt-colon.cc (tree_colon_expression::make_range): Force creation of range if expression is a for command expression. * basics.txi, numbers.txi: Document changes.