Make the TLSv1.3 downgrade mechanism a configurable option

Make it disabled by default. When TLSv1.3 is out of draft we can remove
this option and have it enabled all the time.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3022)
diff --git a/Configure b/Configure
index b7d669c..191fe73 100755
--- a/Configure
+++ b/Configure
@@ -407,6 +407,7 @@
     "tests",
     "threads",
     "tls",
+    "tls13downgrade",
     "ts",
     "ubsan",
     "ui",
@@ -451,6 +452,7 @@
                   "ubsan"		=> "default",
           #TODO(TLS1.3): Temporarily disabled while this is a WIP
 		  "tls1_3"              => "default",
+		  "tls13downgrade"      => "default",
 		  "unit-test"           => "default",
 		  "weak-ssl-ciphers"    => "default",
 		  "zlib"                => "default",