Currently <WLAN>.isconnected() always returns True if a static IP is set,
regardless of the state of the connection.
This patch introduces a new flag 'wifi_sta_connected' which is set in
event_handler() when GOT_IP event is received and reset when DISCONNECTED
event is received (unless re-connect is successful). isconnected() now
simply returns the status of this flag (for STA_IF).
The pre-existing flag misleadingly named 'wifi_sta_connected" is also
renamed to 'wifi_sta_connect_requested'.
Fixes issue #3837
039f196c56