Support IPv6's `nextheader` field

[qdeslandes]

An IPv6 packet's nextheader field can contain one of the following information:

• Layer 4 protocol identifier
• IPv6 extension header identifier

An IPv6 extension header will be located after the IPv6 header. There could be one or more extension header before the layer 4 header.

The current packet preprocessing logic defined by bpfilter always assume nextheadr contains the layer 4 header, meaning IPv6 packets containing extension headers are not properly preprocessed.