MINOR: ssl: set SSL_OP_PRIORITIZE_CHACHA

Sets OpenSSL 1.1.1's SSL_OP_PRIORITIZE_CHACHA unconditionally, as per [1]:

When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
ChaCha20-Poly1305 cipher is at the top of the client cipher list. This
helps those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher
is anywhere in the server cipher list; but still allows other clients to
use AES and other ciphers. Requires SSL_OP_CIPHER_SERVER_PREFERENCE.

[1] https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_clear_options.html

doc/configuration.txt

@@ -10961,6 +10961,9 @@ prefer-client-ciphers
   Use the client's preference when selecting the cipher suite, by default
   the server's preference is enforced. This option is also available on
   global statement "ssl-default-bind-options".
+  Note that with OpenSSL >= 1.1.1 ChaCha20-Poly1305 is reprioritized anyway
+  (without setting this option), if a ChaCha20-Poly1305 cipher is at the top of
+  the client cipher list.
 
 process <process-set>[/<thread-set>]
   This restricts the list of processes and/or threads on which this listener is

src/ssl_sock.c

@@ -1957,6 +1957,9 @@ ssl_sock_generate_certificate_from_conn(struct bind_conf *bind_conf, SSL *ssl)
 #ifndef SSL_MODE_SMALL_BUFFERS                          /* needs small_records.patch */
 #define SSL_MODE_SMALL_BUFFERS 0
 #endif
+#ifndef SSL_OP_PRIORITIZE_CHACHA                        /* needs OpenSSL >= 1.1.1 */
+#define SSL_OP_PRIORITIZE_CHACHA 0
+#endif
 
 #if (OPENSSL_VERSION_NUMBER < 0x1010000fL)
 typedef enum { SET_CLIENT, SET_SERVER } set_context_func;
@@ -3711,6 +3714,7 @@ ssl_sock_initial_ctx(struct bind_conf *bind_conf)
 		SSL_OP_SINGLE_DH_USE |
 		SSL_OP_SINGLE_ECDH_USE |
 		SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION |
+		SSL_OP_PRIORITIZE_CHACHA |
 		SSL_OP_CIPHER_SERVER_PREFERENCE;
 	long mode =
 		SSL_MODE_ENABLE_PARTIAL_WRITE |