lwip2: stability and time management

[d-a-v]

sync with upstream lwip2:
expose sntp_set_system_time() for external RTC without NTP server
fix #1679 (time() works from start)
fix #2505 (honour DST in configTime())
fix lwip2 crashing with WiFi.softAPConfig(ip,ip,ip)

[d-a-v]

I found nowhere in the docs where configTime() is mentionned.
I would be worth mentionning sntp_set_system_time() which allows to use an external RTC.
This function has always been existing but was not exposed.

Current version of lwipopts (v2) allows only one ntp server. Should it allows 3 servers (as per configTime()) ?

Also, timezone and DST are only taken into account when sntp_set_system_time() is called, which can happen:

• by user
• by configTime() (but takes effect once an IP address is setup )
• by dhcp client (if dhcp server provides ntp server)

Currently the API does not let the user decide to refuse the ntp server provided by dhcp (ntp was not fed by dhcp with lwip1.4).

[Juppit]

configTime() filles only the structures in sntp.h
Note that sntp_set_daylight() is still not in sntp.h where it supposed to be.

[d-a-v]

You are perfectly right about sntp_set_daylight() missing declaration. I just realized that we have also Wno-implicit-function-declaration in gcc options. We would then have to declare millis(), micros() too. I think this should go to another PR.
This PR is all about lwip2 currently only in master. configTime() calls sntp_set_daylight() which is implemented there.

edit: previous comment fixed about configTime()