cat: Fix immediate output of processed data

cat: Fix immediate output of processed data

Introduced by commit 790892db, 2006-06-08 "Ensure that cat works ...".
* NEWS: Mention the bugfix.
* src/cat.c (cat): Fix the typo which stopped the writing
of processed data before a blocking read() is done.
* tests/misc/cat-buf: Add to ensure processed data is not buffered.
* tests/Makefile.am: Reference the new test.