Power on screenmode can not be set to 10

[antibyte]

4: Power-on screen mode [1 (default) to 10] Sets the screen mode the system defaults to at power-on.

config 4,10 Illegal value (from 1 to 9)

[uli]

Off-by-one because screen modes start at 1...

[Hysterical raisins: 0 used to be serial console in ttbasic, but we don't support that for lack of pins.]