Implement FD_CLOEXEC

[afxgroup]

When you have a piece of code like this:

/* Make sure we close the file descriptor on exec.  */
int flags = fcntl (fileno (mallstream), F_GETFD, 0);
if (flags >= 0) {
  flags |= FD_CLOEXEC;
  fcntl (fileno (mallstream), F_SETFD, flags);
}

We need to take care on file destructor to close the stream when an exe is spawned.

in init_exit.c on stdio folder there is a test when to close a file:

if (FLAG_IS_SET(__clib4->__fd[i]->fd_Flags, FDF_IN_USE) && FLAG_IS_CLEAR(__clib4->__fd[i]->fd_Flags, FDF_NO_CLOSE)) {

We can add a test on FDF_CLOEXEC there. However FDF_CLOEXEC is only set on pipe2 function and needs to be set also on fcntl