stdbuf: make it mandatory to specify a buffering option

This is consistent with the documented interface and
avoids any ambiguity in a user thinking that stdbuf without options
might reset to a "standard" buffering setup.

* src/stdbuf.c (set_libstdbuf_options): Indicate with the return value
whether any env variables were actually set.
(main): Fail unless some env variables were set.
* tests/misc/stdbuf.sh: Ensure this constraint is enforced.
* NEWS: Mention the small change in behavior.

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