split: Add missing test cases.

This adds test cases for c4f7198f47c1 and ac17fc816e67.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41755

(cherry picked from commit cbea5eddb1b6ae2a101beacf00bf165517f4f2b1)

split: Code cleanup.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41756

(cherry picked from commit 851bf856d1a7ad5427ea9c22639ab6f6c4a3f503)

split: Fix linecount parsing.

The “undocumented kludge” which unfortunately can't be dropped for backward compatibility reasons was prone to segfaulting and would improperly allow a new linecount when one was already set. Fix these issues and add regression tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41757

(cherry picked from commit 117c54a78ccd214c236806721f21da750e512d3e)