Elevate server TLS cipher preferences over client

The server cipher list is (thanks to #11513) chosen systematically to
put the best choices for Tor first.  The client cipher list is chosen
to resemble a browser.  So let's set SSL_OP_CIPHER_SERVER_PREFERENCE
to have the servers pick according to their own preference order.

changes/ticket11528

@@ -0,0 +1,6 @@
+  o Minor features:
+    - Servers now trust themselves to have a better view than clients of
+      which TLS ciphersuites to choose. (Thanks to #11513, the server
+      list is now well-considered, whereas the client list has been
+      chosen mainly for anti-fingerprinting purposes.) Resolves ticket
+      11528.

src/common/tortls.c

@@ -1261,6 +1261,10 @@ tor_tls_context_new(crypto_pk_t *identity, unsigned int key_lifetime,
     goto error;
   SSL_CTX_set_options(result->ctx, SSL_OP_NO_SSLv2);
 
+  /* Prefer the server's ordering of ciphers: the client's ordering has
+  * historically been chosen for fingerprinting resistance. */
+  SSL_CTX_set_options(result->ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);
+
   /* Disable TLS1.1 and TLS1.2 if they exist.  We need to do this to
    * workaround a bug present in all OpenSSL 1.0.1 versions (as of 1
    * June 2012), wherein renegotiating while using one of these TLS