data: tweaks to next keyboard map

Remove the F5/F6 mappings, since it is wierd to have mappings for
those two but not the other Fn. This suggests a mistake or a large
data gap.

Add the various modifier keys as found in

  https://github.com/NetBSD/src/blob/trunk/sys/arch/next68k/dev/wskbdmap_next.c

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>