dd: new option, status=none to suppress output statistics

dd: new option, status=none to suppress output statistics

* src/dd.c (STATUS_NONE): A new bitmask combining all STATUS_
options, thus used to suppress all informational output.
(struct symbol_value statuses): Expose the "none" option,
corresponding to the STATUS_NONE bitmask above.
(print_stats): Return early if STATUS_NONE is specified.
Also move the call to gethrxtime() down so that it's only
called when needed.
(usage): Describe the new options.
* doc/coreutils.texi (dd invocation): Likewise.
* NEWS: Mention the new feature.
* tests/dd/misc.sh: Ensure the new option works.