ENR structure: Add tcp6, quic6 and udp6.

As discussed in ACDC ethereum#139.

Author: nalepae

specs/phase0/p2p-interface.md

@@ -959,9 +959,9 @@ The Ethereum Node Record (ENR) for an Ethereum consensus client MUST contain the
 The ENR MAY contain the following entries:
 
 -  An IPv4 address (`ip` field) and/or IPv6 address (`ip6` field).
--  A TCP port (`tcp` field) representing the local libp2p TCP listening port.
--  A QUIC port (`quic` field) representing the local libp2p QUIC (UDP) listening port.
--  A UDP port (`udp` field) representing the local discv5 listening port.
+-  An IPv4 TCP port (`tcp` field) representing the local libp2p TCP listening port and/or the corresponding IPv6 port (`tcp6` field).
+-  An IPv4 QUIC port (`quic` field) representing the local libp2p QUIC (UDP) listening port and/or the corresponding IPv6 port (`quic6` field).
+-  An IPv4 UDP port (`udp` field) representing the local discv5 listening port and/or the corresponding IPv6 port (`udp6` field).
 
 Specifications of these parameters can be found in the [ENR Specification](http://eips.ethereum.org/EIPS/eip-778).