split(1): auto-extend suffix length if required

If the input cannot be split into the number of files resulting from the
default suffix length, automatically extend the suffix length rather
than bailing out with 'too many files'.

Suffixes are extended such that the resulting files continue to sort
lexically and "cat *" would reproduce the input. For example, splitting
a 1M lines file into (default) 1000 lines per file would yield files
named 'xaa', 'xab', ..., 'xyy', 'xyz', 'xzaaa', 'xzaab', ..., 'xzanl'.

If '-a' is specified, the suffix length is not auto-extended.

This behavior matches GNU sort(1) since around version 8.16.

Reviewed by: christos
Approved by: kevans
Different Revision: https://reviews.freebsd.org/D38279