Fix untranslatable composition of sentences.

Reported by Goran Uddeborg at
<http://lists.gnu.org/archive/html/bug-bison/2008-06/msg00000.html>.
* THANKS (Goran Uddeborg): Add.
* src/reduce.c (reduce_print): Report the number of nonterminals and
rules useless in the grammar in separate sentences.
* tests/reduce.at (Useless Rules): Update output.
(Reduced Automaton): Likewise.
(Underivable Rules): Likewise.
(Empty Language): Likewise.