Investigate pktinfo interface index issues

[camshaft]

In #907, we removed the local interface index and no longer set it as part of the pktinfo cmsg. In theory, its use should yield some performance since the OS doesn't have to do a reverse lookup of the NIC through the local IP. In practice, unfortunately, in certain environments it leads to packets being dropped without an error or logging.

We should do a deeper dive and figure out if this can be re-enabled again.