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;
 }

testcode/lib/atari/mem.c

@@ -11,7 +11,6 @@
 
 extern int getsp(void);                         /* comes from ../getsp.s */
 
-extern char _dos_type;                          /* bss variable */
 unsigned char data = 0x12;                      /* data variable */
 
 unsigned int *APPMHI = (unsigned int *)14;      /* 14,15 */
@@ -42,6 +41,6 @@ int main(void)
   printf("  sp:              $%04X  (stack ptr)\n", getsp());
 
   if (allocmem) free(allocmem);
-  if (_dos_type != 1) cgetc();
+  if (! _is_cmdline_dos()) cgetc();
   return(0);
 }