Atari: add new function '_is_cmdline_dos()' and some other small changes.

- 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

testcode/lib/atari/defdev.c

@@ -13,6 +13,6 @@ extern char _defdev[];
 int main(void)
 {
         printf("default device: %s\n", _defdev);
-        if (_dos_type != SPARTADOS && _dos_type != OSADOS) cgetc();
+        if (! _is_cmdline_dos()) cgetc();
         return 0;
 }