MOS bug fixes

[Tom Seddon]

These would be perfect for feature flags:

OSBYTE $6b (MOS 3.20 only):

acorn_mos_disassembly/src/mos.s65

Line 9984 in 01d707a

 lda #ACCCON.X                ; BUG! This should be TYA 

QnD Y2K fix, so it's always 20xx:

acorn_mos_disassembly/src/rtc.s65

Line 467 in ac672c5

 lda #$19                     ;it's always 19xx... right? 

(should be lda #$20)

*MOVE permissions bug:

acorn_mos_disassembly/src/terminal.s65

Line 1602 in 01d707a

 lda #$F0                     ; Mask out 'public' access bits 

(should be lda #$20) (see JGH's BuildMOS: https://mdfs.net/Info/Comp/BBC/SROMs/BuildMOS)