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

doc/atari.sgml

@@ -275,6 +275,7 @@ See the <url url="funcref.html" name="function reference"> for declaration and u
 <item>_getcolor
 <item>_getdefdev
 <item>_graphics
+<item>_is_cmdline_dos
 <item>_rest_vecs
 <item>_save_vecs
 <item>_scroll