tls: implement clientCertEngine option

Add an option 'clientCertEngine' to `tls.createSecureContext()` which gets
wired up to OpenSSL function `SSL_CTX_set_client_cert_engine`. The option
is passed through from `https.request()` as well. This allows using a custom
OpenSSL engine to provide the client certificate.

lib/https.js

@@ -160,6 +160,10 @@ Agent.prototype.getName = function getName(options) {
   if (options.cert)
     name += options.cert;
 
+  name += ':';
+  if (options.clientCertEngine)
+    name += options.clientCertEngine;
+
   name += ':';
   if (options.ciphers)
     name += options.ciphers;