Add X25519 to the default set of curves

Since many clients default to the X25519 curve in the TLS handshake,
the fact that the server by defualt doesn't support it cause an extra
roundtrip for each TLS connection.  By adding multiple curves, which
is supported since 3d1ef3a15c, we can reduce the risk of extra
roundtrips.

Author: Daniel Gustafsson <daniel@yesql.se>
Co-authored-by: Jacob Champion <jacob.champion@enterprisedb.com>
Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/20240616234612.6cslu7nqexquvwj7@awork3.anarazel.de

doc/src/sgml/config.sgml

@@ -1520,7 +1520,7 @@ include_dir 'conf.d'
         It does not need to be the same curve used by the server's Elliptic
         Curve key.  This parameter can only be set in the
         <filename>postgresql.conf</filename> file or on the server command line.
-        The default is <literal>prime256v1</literal>.
+        The default is <literal>X25519:prime256v1</literal>.
        </para>
 
        <para>

src/backend/utils/misc/guc_tables.c

@@ -4768,7 +4768,7 @@ struct config_string ConfigureNamesString[] =
 		},
 		&SSLECDHCurve,
 #ifdef USE_SSL
-		"prime256v1",
+		"X25519:prime256v1",
 #else
 		"none",
 #endif

src/backend/utils/misc/postgresql.conf.sample

@@ -114,7 +114,7 @@
 #ssl_ciphers = 'HIGH:MEDIUM:+3DES:!aNULL'	# allowed TLSv1.2 ciphers
 #ssl_tls13_ciphers = ''	# allowed TLSv1.3 cipher suites, blank for default
 #ssl_prefer_server_ciphers = on
-#ssl_groups = 'prime256v1'
+#ssl_groups = 'X25519:prime256v1'
 #ssl_min_protocol_version = 'TLSv1.2'
 #ssl_max_protocol_version = ''
 #ssl_dh_params_file = ''

src/test/ssl/t/SSL/Server.pm

@@ -301,7 +301,7 @@ sub switch_server_cert
 	$node->append_conf('sslconfig.conf', "ssl=on");
 	$node->append_conf('sslconfig.conf', $backend->set_server_cert(\%params));
 	# use lists of ECDH curves and cipher suites for syntax testing
-	$node->append_conf('sslconfig.conf', 'ssl_groups=prime256v1:secp521r1');
+	$node->append_conf('sslconfig.conf', 'ssl_groups=X25519:prime256v1:secp521r1');
 	$node->append_conf('sslconfig.conf',
 		'ssl_tls13_ciphers=TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256');