Atari, Atari5200: disable "attract mode" on mouse or joystick input

Fixes #736.

libsrc/atari5200/joy/atr5200std.s

@@ -82,6 +82,7 @@ SENSIVITY       = 16
 
 READJOY:
         and     #3              ; put joystick number in range, just in case
+        sta     jsnum           ; remember joystick number
         tay
         asl     a
         tax                     ; Joystick number * 2 (0-6) into X, index into ZP shadow registers
@@ -89,7 +90,7 @@ READJOY:
         lda     #0              ; Initialize return value
         cmp     TRIG0,y
         bne     @notrg
-        lda     #$10            ; JOY_BTN
+        ora     #$10            ; JOY_BTN
 
 ; Read joystick
 
@@ -119,4 +120,15 @@ READJOY:
 
         ora     #2              ; JOY_DOWN
 
-@done:  rts
+@done:  ldx     #0
+        ldy     jsnum
+        cmp     oldval,y
+        beq     @ret
+        sta     oldval,y
+        stx     ATRACT
+@ret:   rts
+
+.bss
+
+oldval:.res     4
+jsnum: .res     1