split: auto set suffix len for --numeric-suffixes=<N --number=N

split: auto set suffix len for --numeric-suffixes=<N --number=N

Supporting `split --numeric-suffixes=1 -n100` for example.

* doc/coreutils.texi (split invocation): Mention the two
use cases for the FROM parameter, and the consequences on
the suffix length determination.
* src/split.c (set_suffix_length): Use the --numeric-suffixes
FROM parameter in the suffix width calculation, when it's
less than the number of files specified in --number.
* tests/split/suffix-auto-length.sh: Add test cases.
Fixes http://bugs.gnu.org/20511