There is no mechanism to do that at the moment - SSL_set_bio makes changes
to the wbio even if you pass in SSL_get_wbio().

This commit introduces two new API functions SSL_set_rbio() and
SSL_set_wbio(). These do the same job as SSL_set_bio() except they enable
you to manage the rbio and wbio individually.
Reviewed-by: NTim Hudson <tjh@openssl.org>