Never expose ssl->bbio in the public API.

This is adapted from BoringSSL commit 2f87112b963.

This fixes a number of bugs where the existence of bbio was leaked in the
public API and broke things.

- SSL_get_wbio returned the bbio during the handshake. It must always return
  the BIO the consumer configured. In doing so, some internal accesses of
  SSL_get_wbio should be switched to ssl->wbio since those want to see bbio.

- The logic in SSL_set_rfd, etc. (which I doubt is quite right since
  SSL_set_bio's lifetime is unclear) would get confused once wbio got
  wrapped. Those want to compare to SSL_get_wbio.

- If SSL_set_bio was called mid-handshake, bbio would get disconnected and
  lose state. It forgets to reattach the bbio afterwards. Unfortunately,
  Conscrypt does this a lot. It just never ended up calling it at a point
  where the bbio would cause problems.

- Make more explicit the invariant that any bbio's which exist are always
  attached. Simplify a few things as part of that.

RT#4572

Reviewed-by: Richard Levitte <levitte@openssl.org>