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