dd: add support for opening files in Concurrent I/O (CIO) mode

dd: add support for opening files in Concurrent I/O (CIO) mode

* src/dd.c (O_CIO): New flag.
* src/dd.c (O_FULLBLOCK): Add O_CIO to the list of flags that
O_FULLBLOCK should be greater than.
* src/dd.c (flags): Give the name "cio" to the new O_CIO flag, mirroring
the treatment of O_DIRECT.
* src/dd.c (usage): Add a description of the new flag when it is available.
* doc/coreutils.text (dd invocation): Describe the new flag.
* NEWS: Mention the new feature.