Apply --prefix to C++ skeletons too: they change the namespace.

Apply --prefix to C++ skeletons too: they change the namespace.
The test suite already exercize these cases.
* data/c++.m4 (b4_namespace): New.
* data/lalr1.cc, data/glr.cc, data/location.cc: Use it instead of `yy'.
* data/lalr1.cc (yytnameerr_): Move its definition into the namespace.
* data/yacc.c, data/glr.c: Remove a useless `[]'.
* doc/bison.texinfo: Document it.
(Option Cross Key): Use @multitable in all formats.  It looks
nicer, even in TeX outputs.
(Rules): Use the same code whatever the output type is.
* tests/local.at (_AT_BISON_OPTION_PUSHDEFS)
(_AT_BISON_OPTION_POPDEFS): Support AT_NAME_PREFIX.
* tests/calc.at: Use it, instead of hard coding `yy'.