gsocket: add g_socket_send_messages()

Allows sending of multiple messages (packets, datagrams)
in one go using sendmmsg(), thus drastically reducing the
number of syscalls when sending out a lot of data, or when
sending out the same data to multiple recipients.

https://bugzilla.gnome.org/show_bug.cgi?id=719646

gio/giotypes.h

@@ -425,6 +425,41 @@ struct _GOutputVector {
   gsize size;
 };
 
+/**
+ * GOutputMessage:
+ * @address: (allow-none): a #GSocketAddress, or %NULL
+ * @vectors: pointer to an array of output vectors
+ * @num_vectors: the number of output vectors pointed to by @vectors.
+ * @bytes_sent: initialize to 0. Will be set to the number of bytes
+ *     that have been sent
+ * @control_messages: (array length=num_control_messages) (allow-none): a pointer
+ *   to an array of #GSocketControlMessages, or %NULL.
+ * @num_control_messages: number of elements in @control_messages.
+ *
+ * Structure used for scatter/gather data output when sending multiple
+ * messages or packets in one go. You generally pass in an array of
+ * #GOutputVectors and the operation will use all the buffers as if they
+ * were one buffer.
+ *
+ * If @address is %NULL then the message is sent to the default receiver
+ * (as previously set by g_socket_connect()).
+ *
+ * Since: 2.44
+ */
+typedef struct _GOutputMessage GOutputMessage;
+
+struct _GOutputMessage {
+  GSocketAddress         *address;
+
+  GOutputVector          *vectors;
+  guint                   num_vectors;
+
+  guint                   bytes_sent;
+
+  GSocketControlMessage **control_messages;
+  guint                   num_control_messages;
+};
+
 typedef struct _GCredentials                  GCredentials;
 typedef struct _GUnixCredentialsMessage       GUnixCredentialsMessage;
 typedef struct _GUnixFDList                   GUnixFDList;