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create_{read|write}_xdf accept fd instead of filename

Move the file opening from the create_{read|write}_xdf to xdf_open.
Add closefd_ondestroy member to control fd closing

Add closefd_ondestroy to struct xdffile. If non zero, the file descriptor
used to storage will be close when xdf_close is called.
Add xdf_fdopen API function

This functions allows to use a file descriptor instead of a filename in use
of xdffileio opening. At opening, the file descriptor is tested for being
compatible with the mode.
Add fcntl replacement from gnulib

fcntl function is necessary in xdf_fdopen.
Remove open_cloexec (replaced by open)

gnulib module fcntl-h provide all replacements necessary for using O_CLOEXEC
when openning file. Thus it renders open_cloexec and configure check
useless.
Add XDF_CLOSEFD flag to be used by xdf_fdopen

If XDF_CLOSEFD is applied to the mode argument of xdf_fdopen, the file
descriptor is closed when xdf_close is called.
Implement copy_xdf test with xdf_fdopen
Document xdf_fdopen in xdf_open manpage
prefer xz tarball and quiet tests