(cwrite): Change type of `bytes' parameter to size_t

(cwrite): Change type of `bytes' parameter to size_t
Remove now-useless cast.
(stdread): Remove function.
(bytes_split): Use size_t instead of int.
Use safe_read, not stdread.
(lines_split): Likewise.
Use memchr rather than a `while' loop.
(line_bytes_split): Use size_t instead of int.
Use safe_read, not stdread.
(main): Add some FIXME comments to remind me to remove casts.