MFC r295749:

Reorganize the handling all-zeroes terminal block in sparse mode

PR: 189284
(original PR whose fix introduced this bug)

PR: 207092

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

/*- or .\"- or #- to begin license clauses.

Add a test-target and reference vectors for the character converions.