Support zerocopy + bugfix

* NewPacket2 supported DLT_EN10MB packets, and DLT_LINUX_SLL seemed to work most of the time... 95% of the time in a test system, with 5% packet loss in live tests.
* LoopWithCallback as an alternative to NextEx2.  It enables the zerocopy path of libpcap 1.5.0+, even if it's a little more cumbersome to use.

ping @ElPeque 
Just a comment: I remember at some point i had removed a lot of pointers and just allowed values to be copied. I know it would use more total memory, but allocations would be mostly on the stack, thus sending less stuff to the garbage collector.

I had profiled some small improvements when i did that, but of course, if you end up taking a reference somewhere else, the values might be put on the heap anyway and the supposed gain is lost.

So i'm just saying, in some cases, passing by value might be a good idea. The copy is shallow, so the buffer is not going to be copied anyway.