blkzone: (man) reduce two overblown tables to legible proportions

blkzone: (man) reduce two overblown tables to legible proportions

Before version 2.37, before asciidoctor was used, the first three lines
of the first table looked like this:

       start     Zone start sector
       len       Zone length in number of sectors
       wptr      Zone write pointer position

After the move to asciidoctor, the first three lines of the first table
in the generated man page looked like this:

       ┌────────┬────────────────────────────┐
       │        │                            │
       │start   │ Zone start sector          │
       ├────────┼────────────────────────────┤
       │        │                            │
       │len     │ Zone length in number of   │
       │        │ sectors                    │
       ├────────┼────────────────────────────┤
       │        │                            │
       │wptr    │ Zone write pointer         │
       │        │ position                   │
       ├────────┼────────────────────────────┤

Three to four screen lines for just one line of information!

Reduce the tables to sane proportions again by simply using blocks
of preformatted text.

(The substition of xN for x? is needed because somehow the ? in the
indented text is a special character in groff.)

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>