Atari: fix lookup of default device on XDOS.

Stefan Dorndorf, author of XDOS, pointed out that retrieving the
default device by looking at an undocumented memory location won't
work in future XDOS versions.
He also showed a way to get the default device in a compatible
manner.

This change implements his method and adds a version check (XDOS
versions below 2.4 don't support this -- for them the behaviour
will be the same as, for example, AtariDOS: no notion of a default
drive).

libsrc/atari/shadow_ram_handlers.s

@@ -22,6 +22,7 @@ SHRAM_HANDLERS  =       1
         .export         CIO_handler
         .export         SIO_handler
         .export         SETVBV_handler
+        .export         XMOVE_handler
 
 BUFSZ           =       128     ; bounce buffer size
 BUFSZ_SIO       =       256
@@ -1085,6 +1086,24 @@ SETVBV_handler:
         plp
         rts
 
+;---------------------------------------------------------
+
+XMOVE_handler:
+
+        pha
+        lda     PORTB
+        sta     cur_XMOVE_PORTB
+        enable_rom
+        pla
+        jsr     XMOVE_org
+        php
+        pha
+        disable_rom_val cur_XMOVE_PORTB
+        pla
+        plp
+        rts
+
+
 CIO_a:                  .res    1
 CIO_x:                  .res    1
 CIO_y:                  .res    1
@@ -1093,6 +1112,7 @@ cur_CIOV_PORTB:         .res    1
 cur_SIOV_PORTB:         .res    1
 cur_KEYBDV_PORTB:       .res    1
 cur_SETVBV_PORTB:       .res    1
+cur_XMOVE_PORTB:        .res    1
 orig_ptr:               .res    2
 orig_len:               .res    2
 req_len:                .res    2