ipv4: improve check for icmp errors
ipv4: rename Reserved flag to Evil as per RFC3514
packet: rename name tag to string

Just cosmetics...
packet: rename PayloadType() method to GuessPayloadType()
tcp/udp: return Raw as guessed payload type by default
icmpv4: set payload only with the appropriate types
layers: add FindLayer() method
layers: make UnpackAll() return a single packet instead of a slice