[inetstack] TcpSegment struct used for transmit requests expects a payload buffer

[BrianZill]

Currently, the TcpSegment structure has a "data" field of type Buffer, which is used to specify the packet payload data to be sent. This field is not optional, so even when sending a header-only packet (e.g. ACKs), an empty buffer has to be created to fill this field.

To avoid extra overhead around creating empty buffers, it would be nice to make this field optional, by changing its type to Option<Buffer>.