gio/tests/socket: add test for g_socket_send_message()
gio/tests/socket: add datagram version of test_ip_sync
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/tests/socket: add unit test for g_socket_send_messages()

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