Add netmap support for tun(4) interfaces

[giuseppe.lettieri_unipi.it]

Summary

This patch adds a couple of functions and hooks to allow the generic netmap driver to intercept packets coming from a tun(4) interface. The functionality is needed by people using netmap to interpose, e.g., Sensei or Suricata between VPN/PPPoE connections and the host stack.

Test Plan

Currently being tested by Sunny Valley Networks.

[kevans]

Hi, can you please amend this review with the full context? (e.g. git diff -U99999)

Thanks!

[giuseppe.lettieri_unipi.it]

Full context diff