Drop support for OpenSSL < 1.1.1

As of 1 January 2020, there will be no supported OpenSSL version earlier than version 1.1.1. (See https://www.openssl.org/policies/releasestrat.html)

We can clean up our code by dropping backward-compatibility support for earlier versions.

As we do this, we should test with supported versions of LibreSSL as well, since we try to support that too.