variables: use singular in %define variable names

variables: use singular in %define variable names

See http://lists.gnu.org/archive/html/bison-patches/2012-02/msg00045.html

* doc/bison.texi, src/lalr.c, src/main.c, src/muscle-tab.c,
* src/print.c, src/reader.c, src/tables.c, tests/conflicts.at,
* tests/input.at, tests/reduce.at:
s/lr.default-reductions/lr.default-reduction/
s/lr.keep-unreachable-states/lr.keep-unreachable-state/.
* NEWS: Document.