gsocket: Enable TCP_NODELAY by default for stream GSockets

`TCP_NODELAY` disables Nagle’s algorithm, which is generally a better
default for modern networks than having it enabled. Nagle’s algorithm
delays sending small data blobs until they fill an entire TCP segment,
so as to amortise the cost of sending the segment.

This improves bandwidth at the cost of latency. Given the large
bandwidth capabilities of most modern networks, most streams are
constrained by latency rather than bandwidth, so disabling Nagle’s
algorithm makes sense.

Various other major bits of software (such as libcurl) already disable
Nagle’s algorithm by default.

Specific applications which need it can turn it back on by calling
`g_socket_set_option()`.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #791

gio/gsocket.c

@@ -138,6 +138,24 @@
  * a `GSocket` concurrently from multiple threads, you must implement your own
  * locking.
  *
+ * ## Nagle’s algorithm
+ *
+ * Since GLib 2.80, `GSocket` will automatically set the `TCP_NODELAY` option on
+ * all `G_SOCKET_TYPE_STREAM` sockets. This disables
+ * [Nagle’s algorithm](https://en.wikipedia.org/wiki/Nagle%27s_algorithm) as it
+ * typically does more harm than good on modern networks.
+ *
+ * If your application needs Nagle’s algorithm enabled, call
+ * [method@Gio.Socket.set_option] after constructing a `GSocket` to enable it:
+ * ```c
+ * socket = g_socket_new (…, G_SOCKET_TYPE_STREAM, …);
+ * if (socket != NULL)
+ *   {
+ *     g_socket_set_option (socket, IPPROTO_TCP, TCP_NODELAY, FALSE, &local_error);
+ *     // handle error if needed
+ *   }
+ * ```
+ *
  * Since: 2.22
  */
 
@@ -749,6 +767,8 @@ g_socket_constructed (GObject *object)
       /* See note about SIGPIPE below. */
       g_socket_set_option (socket, SOL_SOCKET, SO_NOSIGPIPE, TRUE, NULL);
 #endif
+      if (socket->priv->type == G_SOCKET_TYPE_STREAM)
+        g_socket_set_option (socket, IPPROTO_TCP, TCP_NODELAY, TRUE, NULL);
     }
 }