Don't exhaust virtual memory when processing large inputs.

Don't exhaust virtual memory when processing large inputs.
Fix this by removing csplit's internal free-list management;
instead rely on malloc for that.

(free_list): Remove global.
(clear_all_line_control): Remove function.
(get_new_buffer): Always use create_new_buffer to obtain a
new buffer, rather than searching free_list.
(free_buffer): Just call free.