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.

lwip/include/arch/cc.h

@@ -39,6 +39,7 @@
 #include <sys/time.h>
 #include <sys/errno.h>
 #include <esp/hwrand.h>
+#include <fcntl.h>
 
 struct ip4_addr;
 struct esf_buf;