LWIP_SOCKET_OFFSET macro

LWIP_SOCKET_OFFSET==n: Increases the file descriptor number created by LwIP with n. FD_SETSIZE from sys/types.h is the maximum number of supported file descriptors and CONFIG_LWIP_MAX_SOCKETS defines the number of sockets; LWIP_SOCKET_OFFSET is configured to use the largest numbers of file descriptors for sockets. File descriptors from 0 to LWIP_SOCKET_OFFSET-1 are non-socket descriptors and from LWIP_SOCKET_OFFSET to FD_SETSIZE are socket descriptors.