API does not provide a way to inject to an inactive handle

[nicknezis]

I am using a Myricom snf library which allows packet injection through an inactive handle. This allows for packet injection without the need for any read buffers to be established. In my use case, I need to inject IGMP packets to subscribe to data which is then read by a different process which is doing the sniffing.

I was able to accomplish this by adding the following method to the InactiveHandle interface. nicknezis@0d08062#diff-69b81a223850b8b6476f978036525756

I'm not sure if there is a better way to accomplish this, but it seems like a limitation of the gopacket abstraction above the lower libpcap library. I'm open to suggestions if there are better approaches (i.e. should there be a third interface type? WriteableInactiveHandle)