Add `verify_fail_if_no_cert` option for proper client authentication

Using `verify_peer` on server side requests a certificate from the
client. If no certificate is provided, the server silently accepts the
connection and rejects if the client sends an unexpected certificate.
Adding `verify_fail_if_no_cert` has no affect on client and for server
requires that the peer sends a certificate or fails the handshake. This
is the desired behavior when the user specifies a fingerprint or CA file.

contrib/epee/src/net_ssl.cpp

@@ -330,7 +330,7 @@ bool ssl_options_t::handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::soc
     socket.set_verify_mode(boost::asio::ssl::verify_none);
   else
   {
-    socket.set_verify_mode(boost::asio::ssl::verify_peer);
+    socket.set_verify_mode(boost::asio::ssl::verify_peer | boost::asio::ssl::verify_fail_if_no_peer_cert);
     socket.set_verify_callback([&](bool preverified, boost::asio::ssl::verify_context &ctx)
     {
       // preverified means it passed system or user CA check. System CA is never loaded