Retire prev_bio

Retire prev_bio

While BIO chains are doubly linked lists, nothing has ever made use of this
fact internally. Even libssl has failed to maintain prev_bio properly in
two places for a long time. When BIO was made opaque, the opportunity to
fix that was missed. Instead, BIO_set_next() now allows breaking the lists
from outside the library, which freerdp has long done.

Problem found by schwarze while trying to document BIO_set_next().

schwarze likes the idea
ok jsing