split: Improve GNU compatibility.
- Like GNU split, turn autoextend back on if given -a0.
- Add a test case that verifies that -a<non-zero> turns autoextend off.
- Add a test case that verifies that -a0 turns autoextend back on.
Sponsored by: Klara, Inc.
Reviewed by: christos, kevans
Differential Revision: https://reviews.freebsd.org/D42011