New code from lwip project

lwip repo:
aa6f6bc patch #8480 Fix handling of dns_seqno wraparound
f1e023a init.c: LWIP_PPP_API needs NO_SYS==0

lwip-contrib repo:
4325348 partly applied patch #8357: fix compiler warnings in tcpdump.c
96e27bf minor: unix port removed unused define IPv4
c6c94d2 minor: tcpdump: fixed source code layout