Support TCP_ACCEPT_FILTER in 3.x

From the commit message for ec1b6dda8e8cbf7663e36546150939925de48a09:

> Since Go will not allow nil where a string is expected, there is no way to change the implementation of existing methods to accept nil strings. This is necessary to properly support e.g. TCP_ACCEPT_FILTER, which can only be cleared by setting it to nil.