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.
mqtt_timer_left_ms: timer->end_time is in ticks, now is in ticks, so left
is in ticks. left * portTICK_PERIOD_MS is the time left.
With that change, the select in both mqtt_esp_read and mqtt_esp_write
seems to work as expected(with time, not ticks)
