etdump.1: Polish + xref cd9660(5)

+ correct misfile in section 8
+ rewrite the document descirption to include -9660
+ markup command modifiers as such instead of bold
+ markup variables as defined variables, enabling apropos
+ correct option syntax in description section
+ unwrap list offsets for legibility
+ fix line wrapping for standard BSD console
+ crossreference cd9660(5) and vis versa
+ tag SPDX for both manuals

MFC after: 3 days
Reviewed by: mhorne
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48513

(cherry picked from commit 283be95ea29abd7f867e4084bafe368c47f6c038)