split: fix problems with /dev/zero

split: fix problems with /dev/zero

Problem reported by Nelson H.F. Beebe in: http://bugs.gnu.org/22624
Other problems also fixed: basically, the code got confused because
GNU/Linux reports that /dev/zero has size zero.
* src/split.c (input_file_size): Now takes struct stat *, not just
size.  Always store the first buffer.  All callers changed.  Treat
/dev/zero as an infinitely-large file, both on GNU/Linux where
fstat and lseek say its size is zero, and on GNU/Hurd where they
say the size is OFF_T_MAX.
(cwrite): Return true on success.
(bytes_split): Don't try to read past EOF, and stop if a write fails.
(lines_rr): Omit stray check for ignorable errno.
(main): Get file size only when n_units > 1, since that's the only
time it is needed.  Defer most of the work to input_file_size.
* tests/split/l-chunk.sh: Adjust tests to match new behavior
on oddball inputs.