OpenSSL version 1.1 has broken its API in quite a few ways. To avoid
having to use ifdef's everywhere, we have implemented the BIO functions
added in version 1.1 ourselves in case we are using the legacy API. We
were implementing them in the header file, though, which doesn't make a
lot of sense, since these functions are only ever being used the the
openssl stream implementation.

Move these functions to the implementation file and mark them static.