Back-patch 9.4-era SSL renegotiation code into 9.3 and 9.2.

This back-patches 9.4 commits 31cf1a1a, 86029b31, and 36a3be65 into
the prior branches, along with relevant bits of b1aebbb6 and 7ce2a45a.
We had foreseen doing this once the code was proven, but that never did
happen, probably because we got sufficiently fed up with renegotiation
to disable it by default.  However, we have to do something now because
the prior code doesn't even compile against OpenSSL 1.1.  Per discussion,
the best solution seems to be to make the older branches look like 9.4.

Discussion: https://postgr.es/m/20047.1492305247@sss.pgh.pa.us