IPv6: DHCP6 & NTP fix (lwIP fix from David J. Fiddes [email protected])

[d-a-v]

Two PRs on lwip2 from David J. Fiddes [email protected]:

• Add support for setting SNTP servers via DHCPv6 d-a-v/esp82xx-nonos-linklayer#25

This bring parity between DHCPv4 and DHCPv6 allowing DHCPv6
servers to specify an SNTP server to use. Only a single SNTP
server slot is allocated to DHCPv6 (the same as v4).

Tests:

Start an esp8266 on a network which only provides IPv6
addresses via DHCPv6 with 0, 1 or 3 SNTP servers
configured
Verify that the SNTP is configured and that time is
correct with an updated IPv6.ino sketch that reports
SNTP servers and the current time

• Restart SNTP client when we have an IPv4 or IPv6 address d-a-v/esp82xx-nonos-linklayer#26

This fixes the restarting of the SNTP client when the network
interface has a valid IP address. This was broken in changeset
9246ef for IPv4.

Note that for IPv6 the SNTP server can take up to 15 seconds to
be used due to the async nature of stateless DHCPv6 and SLAAC.
For IPv4 it takes effect immediately as the address and SNTP
server come in the same packet.

Tests:

Verify operation on dual-stack IPv4/IPv6:
    DHCPv6 only network (no DHCPv4 server present)
    DHCPv4 and DHCPv6 network
Verify operation on IPv4 only stack
Check SNTP restart with LwIP logging enabled
Verify time is requested and received via Wireshark
Verify system time is set