Make packet sources interchangeable

Something like this might be convenient for swapping between different capture backends:

```
type PacketDataReader interface {
PacketDataSource
}

type PacketDataReadCloser interface {
PacketDataReader
io.Closer
}

type ZeroCopyPacketDataReader interface {
ZeroCopyPacketDataSource
}

type ZeroCopyPacketDataReadCloser interface {
ZeroCopyPacketDataReader
io.Closer
}
```

Names chosen to match `io` package.

It looks `pcap/afpacket/pfring`...Close don't have a return value though.