dd: minor fullblock changes

dd: minor fullblock changes

* src/dd.c (O_FULLBLOCK): Define using an enum, not #define.
Derive the value, rather than hard-coding to one that might conflict.
(usage): Mention iflag=fullblock in --help output.
(scanargs): Reset the O_FULLBLOCK bit, so that we don't try to set
an undefined attribute via fcntl (fd, F_SETFL, ...
* tests/dd/misc: Signal framework_failure when necessary.
Use "compare actual expected", so any diffs look "right".
* NEWS (dd): Alphabetize and reword.
* coreutils.texi (dd invocation): Adjust wording.