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).
- use this function instead of directly looking at _dos_type in the included
targetutil and test programs
- fixes/improvements to the Atari runtime library regarding the recently
changed _dos_type values
- libsrc/atari/targetutil/w2cas.c: exit if no filename was entered
- add documentation for the new function
