WiFiSTA - allow using DHCP again after disconnecting static IP

[JAndrassy]

reset _useStaticIp to false in WiFi.disconnect()

[d-a-v]

LGTM

[mcspr]

https://github.com/esp8266/Arduino/blob/1c624dd/doc/esp8266wifi/station-class.rst#config

If you pass all three parameter as 0.0.0.0 (local_ip, gateway and subnet), it will re enable DHCP

In our docs this is mentioned right at the end. Might be more discover-able if moved to the top? Plus, what updated disconnect() behaviour from this PR.

[JAndrassy]

@mcspr
I know about config(0,0,0,0). it looks like a workaround.
Do you think resetting back to DHCP in disconnectis not right? That some sketch may want after invoking disconnect() join the network with same previously configured static IP?

I try to find the common Arduino WiFi API so there are as few differences as possible between various WiFi libraries. But config(0, 0, 0, 0) would be hard 'hard to sell'. Having disconnect() reset to DHCP is much simpler.

[mcspr]

I only thought about docs... But, considering some examples, that definitely breaks something like this? Where user would expect config() to persist and does not to always pair config() and begin(). Not sure how other cores and libs behave here

setup() {
  WiFi.config()
}
loop() {
  WiFi.begin(foo, bar)
  ... work ...
  WiFi.disconnect()
}

For config, might be another method?

config() {
  config(0, 0, 0, 0)
}

[JAndrassy]

I am too tired now, but I will revert this tomorrow. In esp32 too. config(0) gives more control to the user of the library. I will try to steer other libraries to config(0), if they don't support it already.