Fix two issues with the filename creation algorithm. First, this
corrects the behavior of the `-a` option. This commit ensures a
failure occurs when the number of chunks exceeds the number of
filenames representable with the specified fixed width:
$ printf "%0.sa" {1..11} | split -d -b 1 -a 1
split: output file suffixes exhausted
Second, this corrects the behavior of the default behavior when `-a`
is not specified on the command line. Previously, it was always
settings the filenames to have length 2 suffixes. This commit corrects
the behavior to follow the algorithm implied by GNU split, where the
filename lengths grow dynamically by two characters once the number of
chunks grows sufficiently large:
$ printf "%0.sa" {1..91} | ./target/debug/coreutils split -d -b 1 \
> && ls x* | tail
x81
x82
x83
x84
x85
x86
x87
x88
x89
x9000
