Allow C functions/variables with names starting with "__" to be found
from within classes, even though their names would normally be
inaccessible due to the Python "class private names" mechanism.
https://github.com/cython/cython/issues/3544

Warn about unmangled Python names that are being found and used, since they do not adhere to Python lookup rules.

Removed `mangle_special_name` in favour of `mangle_class_private_name`,
which previously applied slightly different rules for creating vs looking up variables.

Closes #3548.