split: add --number to generate a particular number of files

split: add --number to generate a particular number of files

* src/split.c (usage, long_options, main): New options --number,
--unbuffered, --elide-empty-files.
(set_suffix_length): New function to auto increase suffix length
to handle a specified number of files.
(create): New function.  Refactored from cwrite() and ofile_open().
(bytes_split): Add max_files argument to support byte chunking.
(lines_chunk_split): New function.  Split file into chunks of lines.
(bytes_chunk_extract): New function.  Extract a chunk of file.
(of_info): New struct.  Used by functions lines_rr and ofile_open
to keep track of file descriptors associated with output files.
(ofile_open): New function.  Shuffle file descriptors when there
are more output files than available file descriptors.
(lines_rr): New function to distribute lines round-robin to files.
(chunk_parse): New function.  Parses K/N syntax.
* tests/misc/split-bchunk: New test for byte chunking.
* tests/misc/split-lchunk: New test for line delimited chunking.
* tests/misc/split-rchunk: New test for round-robin chunking.
* tests/Makefile.am: Reference new tests.
* tests/misc/split-fail: Add failure scenarios for new options.
* tests/misc/split-l: Fix a typo. s/ln/split/.
* doc/coreutils.texi (split invocation): Document --number.
* NEWS: Mention the new feature.
* .mailmap: Map new email address for shortlog.

Signed-off-by: Pádraig Brady <P@draigBrady.com>