Rename "default rule" to "default reduction".

Rename "default rule" to "default reduction".

This includes changing variable names in code, changing
comments, and renaming %define lr.default_rules to %define
lr.default_reductions.
* NEWS (2.5): Update IELR documentation.
* data/glr.c, data/lalr1.cc, data/lalr1.java, data/yacc.c:
Adjust YYDEFACT and yydefact_ documentation.
* doc/bison.texinfo (Decl Summary): Adjust lr.default_reductions
and lr.type documentation.  Make some other wording
improvements.
(Glossary): Adjust cross-references and Default Reduction
definition.
* src/lalr.c (state_lookahead_tokens_count): Adjust code.
Remove a confusing comment pointed out by Akim Demaille.
(initialize_LA): Adjust code.
* src/print-xml.c (print_reductions): Adjust code.
* src/print.c (print_reductions): Adjust code.
* src/reader.c (reader): Adjust code.
* src/tables.c (action_row): Adjust code.
(token_actions): Adjust code.
* src/tables.h: Adjust YYDEFACT documentation.
* tests/input.at (%define lr.default_rules invalid values):
Rename test group to...
(%define lr.default_reductions invalid values): ... this, and
update grammar file and expected output.
* tests/reduce.at (AT_TEST_LR_DEFAULT_RULES): Rename to...
(AT_TEST_LR_DEFAULT_REDUCTIONS): ... this, and update.