[PATCH] Avoid mixing custom and OpenSSL BIO functions

Hi all,

I've attached a patch for the master branch to fix up the custom BIOs used
by PostgreSQL, in light of the issues with the OpenSSL update recently.
While c82207a548db47623a2bfa2447babdaa630302b9 (switching from BIO_get_data
to BIO_get_app_data) resolved the immediate conflict, I don't think it
addressed the root cause, which is that PostgreSQL was mixing up two BIO
implementations, each of which expected to be driving the BIO.

It turns out the parts that came from the OpenSSL socket BIO were a no-op,
and in fact PostgreSQL is relying on it being a no-op. Instead, it's
cleaner to just define a custom BIO the normal way, which then leaves the
more standard BIO_get_data mechanism usable. This also avoids the risk that
a future OpenSSL will add a now BIO_ctrl to the socket type, with libssl
calling into it, and then break some assumptions made by PostgreSQL.

I've attached a patch which does that. The existing SSL tests pass with
it, tested on Debian stable. (Though it took me a few iterations to figure
out how to run the SSL tests, so it's possible I've missed something.)

The patch is not expected to change behavior, so nothing new to document,
nor any expected performance impact.

David