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).

asminc/atari.inc

@@ -1024,9 +1024,16 @@ XFILE        = $087D    ; XDOS filename buffer
 XLINE        = $0880    ; XDOS DUP input line
 XGLIN        = $0871    ; get line
 XSKIP        = $0874    ; skip parameter
+.ifdef __ATARIXL__
+.ifndef SHRAM_HANDLERS
+.import XMOVE_handler
+.endif
+.define XMOVE XMOVE_handler
+XMOVE_org    = $0877    ; move filename
+.else
 XMOVE        = $0877    ; move filename
+.endif
 XGNUM        = $087A    ; get number
-XDEFDEV      = $0816    ; current drive * undocumented *
 
 ;-------------------------------------------------------------------------
 ; End of atari.inc