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.

asminc/errno.inc

@@ -28,6 +28,7 @@
         ESPIPE                  ; Illegal seek
         ERANGE                  ; Range error
         EBADF                   ; Bad file number
+        ENOEXEC                 ; Exec format error
         EUNKNOWN                ; Unknown OS specific error - must be last!
 
         EMAX    = EUNKNOWN      ; Highest error code