use stack for tracking lexer start states * lex.h (octave_lexer::start_state_stack): New member variable. (octave_lexer::push_start_state, octave_lexer::pop_start_state, octave_lexer::clear_start_state, octave_lexer::start_state, octave_lexer::display_start_state): New functions. * lex.ll (INPUT_FILE_START): Rename from INPUT_FILE_BEGIN. Use push_start_state and pop_start_state instead of switching flex start states directly with the flex BEGIN macro. (octave_lexer::init, octave_lexer::reset): Clear start state stack. (octave_lexer::prep_for_file, octave_lexer::process_comment, octave_lexer::handle_close_bracket): Don't use OCTAVE_YYG macro. (octave_lexer::display_start_state): Rename from display_state. Eliminate argument. Display current start state.