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