Implement exec() for Atari XDOS.

- Adds new ENOEXEC error code, also used by Apple2 targets.
- Maximum command line length is 40, incl. program name. This is
  an XDOS restriction.
- testcode/lib/tinyshell.c has been extended to be able to run
  programs.

include/errno.h

@@ -72,7 +72,8 @@ extern int _errno;
 #define ESPIPE          14      /* Illegal seek */
 #define ERANGE          15      /* Range error */
 #define EBADF           16      /* Bad file number */
-#define EUNKNOWN        17      /* Unknown OS specific error */
+#define ENOEXEC         17      /* Exec format error */
+#define EUNKNOWN        18      /* Unknown OS specific error */