split: be more careful about buffer sizes

split: be more careful about buffer sizes

* src/split.c: Include sys-limits.h, not safe-read.h.
(input_file_size, bytes_split, lines_split, line_bytes_split)
(lines_chunk_split, bytes_chunk_extract, lines_rr): Call read, not
safe_read, since safe_read no longer buys us anything.
(main): Reject outlandish buffer sizes right away,
rather than allocating huge buffers and never using them.