LWIP_SOCKET_EXTERNAL_HEADERS macro

LWIP_SOCKET_EXTERNAL_HEADERS==1: Use external headers instead of sockets.h and inet.h. In this case, user must provide its own headers by setting the values for LWIP_SOCKET_EXTERNAL_HEADER_SOCKETS_H and LWIP_SOCKET_EXTERNAL_HEADER_INET_H to appropriate include file names and the whole content of the default sockets.h and inet.h is skipped.