api.namespace: demonstrate and use {...} values instead of "..." values

api.namespace: demonstrate and use {...} values instead of "..." values

* tests/c++.at, tests/input.at: Use "%define api.namespace {foo}" instead
of using quotes.
* tests/local.at (AT_SETUP_STRIP, AT_NAME_PREFIX): Recognize uses of
braces instead of quotes.
* doc/bison.texi: Use braces for api.namespace's values.