split: support an arbitrary number of split files by default

split: support an arbitrary number of split files by default

* src/split.c (next_file_name): If `suffix_auto' is true and the first
suffix character is 'z', generate a new file file name adding `z' to
the prefix and increasing the suffix length by one.
(set_suffix_length): Disable auto suffix width in various cases.
* tests/split/suffix-auto-length: Test it.
* doc/coreutils.texi (split invocation): Mention it.
* NEWS (Improvements): Likewise.