variables: simplify the upgrade of namespace into api.namespace.

variables: simplify the upgrade of namespace into api.namespace.

This patch simplifies "variables: rename namespace as api.namespace",
commit 67501061076ba46355cfd9f9361c7eed861b389c.
Suggested by Joel E. Denny in
http://lists.gnu.org/archive/html/bison-patches/2009-07/msg00006.html

	* src/muscle-tab.c (muscle_percent_variable_update): New.
	(muscle_percent_define_insert): Use it in replacement of the
	previous tr invocation.
	Remove variable_tr, no longer needed.
	* data/bison.m4 (b4_percent_define_copy_, b4_percent_define_copy):
	Remove.
	* data/c++.m4: No longer handle namespace -> api.namespace.
	* tests/input.at (%define backward compatibility): Check that
	namespace is treated as api.namespace.