Add SNI support for OpenSSL. (see #3923)

Add SNI support for OpenSSL. (see #3923)

The original patch for this is by Phil Pennock at:
https://people.spodhuis.org/phil.pennock/software/mutt-patches/

I have removed the OpenSSL version check and defined(OPENSSL_NO_TLSEXT)
check because:
  * SSL_set_tlsext_host_name() was added in 0.9.8f [11 Oct 2007]
  * OpenSSL 1.1 no longer has the OPENSSL_NO_TLSEXT compilation option
  * https://rt.openssl.org/Ticket/Display.html?id=2788&user=guest&pass=guest
    shows that the no-tlsext compilation option has been broken for some time.
  * Going forward, I'd like to minimize and start removing cruft required
    to support ancient/insecure versions of libraries.