[console] Restore compatibility with "--key" values in existing scripts

[console] Restore compatibility with "--key" values in existing scripts

Commit 3ef4f7e ("[console] Avoid overlap between special keys and
Unicode characters") renumbered the special key encoding to avoid
collisions with Unicode key values outside the ASCII range.  This
change broke backwards compatibility with existing scripts that
specify key values using e.g. "prompt --key" or "menu --key".

Restore compatibility with existing scripts by tweaking the special
key encoding so that the relative key value (i.e. the delta from
KEY_MIN) is numerically equal to the old pre-Unicode key value, and by
modifying parse_key() to accept a relative key value.

Reported-by: Sven Dreyer <sven@dreyer-net.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>