This function (actually a macro in OpenSSL) is used by several projects
(e.g. OpenResty, OpenVPN, ...) so it can useuful to provide it for
compatibility.

However, depending on the semantics of the BIO type (e.g. BIO_pair), the
return value can be meaningless, which might explain why it was removed.

Change-Id: I0e432c92222c267eb994d32b0bc28e999c4b40a7
Reviewed-on: https://boringssl-review.googlesource.com/11020
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>