F_DUPFD_CLOEXEC implementation

One more small change to extend the availability of creation of file
descriptors with FD_CLOEXEC set.  Adding a new command to fcntl() requires
no new system call and the overall impact on code size if minimal.

If this patch gets accepted we will also add this change to the next
revision of the POSIX spec.

To test the patch, use the following little program.  Adjust the value of
F_DUPFD_CLOEXEC appropriately.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#ifndef F_DUPFD_CLOEXEC
# define F_DUPFD_CLOEXEC 12
#endif

int
main (int argc, char *argv[])
{
  if  (argc > 1)
    {
      if (fcntl (3, F_GETFD) == 0)
	{
	  puts ("descriptor not closed");
	  exit (1);
	}
      if (errno != EBADF)
	{
	  puts ("error not EBADF");
	  exit (1);
	}

      exit (0);
    }
  int fd = fcntl (STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
  if (fd == -1 && errno == EINVAL)
    {
      pu...