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

include/atari.h

@@ -161,11 +161,12 @@ extern void __fastcall__ _scroll (signed char numlines);
                                           /* numlines < 0  scrolls down */
 
 /* misc. functions */
-extern unsigned char get_ostype(void);  /* get ROM version */
-extern unsigned char get_tv(void);      /* get TV system */
-extern void _save_vecs(void);           /* save system vectors */
-extern void _rest_vecs(void);           /* restore system vectors */
-extern char *_getdefdev(void);          /* get default floppy device */
+extern unsigned char get_ostype(void);       /* get ROM version */
+extern unsigned char get_tv(void);           /* get TV system */
+extern void _save_vecs(void);                /* save system vectors */
+extern void _rest_vecs(void);                /* restore system vectors */
+extern char *_getdefdev(void);               /* get default floppy device */
+extern unsigned char _is_cmdline_dos(void);  /* does DOS support command lines */
 
 /* global variables */
 extern unsigned char _dos_type;         /* the DOS flavour */