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.

libsrc/common/errormsg.c

@@ -24,6 +24,7 @@ const char* const _sys_errlist[] = {
     "Illegal seek",                 /* ESPIPE */
     "Range error",                  /* ERANGE */
     "Bad file number",              /* EBADF */
+    "Exec format error",            /* ENOEXEC */
     "Unknown OS error code",        /* EUNKNOWN */
 };