https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=269250
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.