New code from lwip project

lwip repo:
382ddac CORE: fixed missing prototype for pbuf_header_impl() function
2b3e020 PPP, fixed some compiler warnings

lwip-contrib repo:
f2e8319 unixsim: fixed various compiler warnings
85121ab unix port: simhost example: updated to include changed PPP files
79b0f6a unix port: fixed tcpdump.c syntax