* upstream lwIP is now downloaded by a makefile, not subsubmoduled
* lwip2: upstream lwIP not sub-sub-modules anymore
lwip2: Allow IPv4 and IPv6 DNS and SNTP server configured via DHCP to co-exist (patch against upstream)
* lwip2: enable tcp-listen-with-backlog feature
* lwip2 submodule update:
- enable more efficient chksum algorithm thanks to Richard Allen
- enable tcp listener with backlog
* more comments, fix backlog management, fix API
* move default value definition in .cpp
because one must not believe it can be redefined before including WiFiServer.h
* improved backlog handling, it is no more a breaking change