col(1): Fix a couple of bugs

- When flushing extra lines after all input has been processed, make
sure that local state is reinitialized correctly.
- When -f is specified, make sure to end output with a full newline.
- Fix some style issues and update comments.
- Add some regression tests.

PR: 249308
Submitted by: Yang Zhong <yzhong@freebsdfoundation.org>
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26536