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

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

The idea of mapping non-existen characters to space (0x20) was nice alone,
but those maps also used as backward maps for Paste, so space becomes mapped
to last non-existen character on Paste as result.

Fix it by mapping non-existen characters to another non-existen one, i.e. to
0x00, so unused 0x00 can be backward-mapped to some junk without real harm.

Pointed by: Alexander Kabaev <ak03@gte.com>

1) Map all unused characters into 'space'.
2) iso-8859-4_to_cp437 not do what its name tells at all, remove it

Generate screen map from source (from 1.1.5.1)