Add the default O_CLOEXEC flag for device file descriptors.  This is
generally considered a safer option as it allows the user a race free
option to decide whether file descriptors are inherited across exec,
with the default avoiding file descriptor leaks.
Reported-by: NYann Droneaud <ydroneaud@opteya.com>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>