[openssl] More improvements to the backend

[openssl] More improvements to the backend

* Don't manually initialise libssl 1.1.0 -- it does this automatically
* SSL_library_init() should be called first otherwise
* Move SSL_CTX construction to rb_setup_ssl_server()
* Test for all required files (certificate & key) before doing anything
* Free the old CTX before constructing a new one (Fixes #186)
* Don't try to set options / ciphers etc on a NULL CTX
* Clean up ifdef indentation
* Fix DH parameters memory leak