api.value.type: use keyword/brace values

api.value.type: use keyword/brace values

Suggested by Joel E. Denny.
http://lists.gnu.org/archive/html/bison-patches/2013-03/msg00016.html

* data/bison.m4 (b4_percent_define_get_kind): New.
(b4_variant_flag): Check that api.value.type is defined as the 'variant'
keyword value.
* data/c.m4 (_b4_value_type_setup_keyword): New.
(b4_value_type_setup): Use it to simplify reading.
Use b4_define_silent.
Decode api.value.type, including its type.
(b4_value_type_define): Likewise.
* data/c++.m4 (b4_value_type_declare): Adjust the decoding of api.value.type,
taking its kind into account.
* doc/bison.texi: Adjust all the examples to the new syntax.
* NEWS: Ditto.
* tests/types.at: Adjust