variables: rename namespace as api.namespace.

Discussed in
http://lists.gnu.org/archive/html/bison-patches/2009-04/msg00033.html

	* data/bison.m4 (b4_percent_define_copy_, b4_percent_define_copy):
	New.
	(b4_percent_define_use): New.
	Use it where applicable.
	* data/c++.m4: Replace uses of the variable "namespace" by
	"api.namespace".
	Default the latter to the former.
	* doc/bison.texinfo (Decl Summary): Document "namespace" as
	obsolete.
	Document api.namespace.
	Use @samp to document %define uses, keep @code for identifiers.
	* NEWS: Likewise.
	* tests/c++.at, tests/input.at: Test api.namespace instead of
	namespace. (The tests passed with namespace.)

tests/c++.at

@@ -303,7 +303,7 @@ m4_define([AT_CHECK_NAMESPACE],
 AT_DATA_GRAMMAR([[input.y]],
 [[%language "C++"
 %defines
-%define namespace "]$1["
+%define api.namespace "]$1["
 %union { int i; }
 %define global_tokens_and_yystype
 %locations