lwip: define LWIP_POSIX_SOCKETS_IO_NAMES as zero (#457)

This patch gets the newlib standard stream descriptors playing well with the
lwip socket descriptors and the spiffs file descriptors. The LWIP_SOCKET_OFFSET
is now defined to be 3, rather than zero, to avoid clashing with the standard
stream descriptors, and the SPIFFS_FILEHDL_OFFSET is bumped up to start after
the lwip descriptors.

examples/http_get/http_get.c

@@ -7,6 +7,7 @@
 #include "espressif/esp_common.h"
 #include "esp/uart.h"
 
+#include <unistd.h>
 #include <string.h>
 
 #include "FreeRTOS.h"

examples/http_get_bearssl/http_get_bearssl.c

@@ -13,6 +13,7 @@
 #include "esp/uart.h"
 #include "esp/hwrand.h"
 
+#include <unistd.h>
 #include <string.h>
 
 #include "FreeRTOS.h"

examples/tls_server_bearssl/tls_server_bearssl.c

@@ -19,6 +19,7 @@
 #include "esp/uart.h"
 #include "esp/hwrand.h"
 
+#include <unistd.h>
 #include <string.h>
 
 #include "FreeRTOS.h"