wc: line-buffer the printed counts

wc: line-buffer the printed counts

* src/wc.c (main): Set stdout to line buffered mode
to ensure parallel running instances don't intersperse
their output.  This adds 6.5% to the run time in the worst case
of many zero length files, but has neglible impact for
standard sized files.
* tests/misc/wc-parallel: New test for atomic output.
* tests/Makefile.am: Reference it.
* NEWS: Mention the fix
This is similar to commit 710fe413, 20-10-2009,
"md5sum, sha*sum, sum: line-buffer the printed checksums"