LwipIntfDev - disconnect()/end() should not clear static IP settings

[JAndrassy]

when I wrote the test sketch for WiFi libraries I wanted to do begin with DHCP, then begin with static IP and then again begin with DHCP. I thought that disconnect() should clear the static IP for return to DHCP, but it turned out that all libraries I tested preserved the static IP settings over disconnect() (except my WiFiEspAT). I learned that the right way to return to DHCP is config(0) or config(INADDR_NONE). And this really worked in all libraries and in modern Ethernet libraries (the ones with config) too.
So the user of the library can config the static IP and then disconnect and later begin again with the same static IP.

Now WiFiTest and ModernEthernetTest do an additional begin/disconnect to test if the library preserves the static IP over disconnect. And they reported the your library resets the static IP in disconnect()/end().

The fix is easy and with the previous PR with the config with one and two parameters, the config(INADDR_NONE) works to return to DHCP.

The tests are:
WiFiTest
ModernEthernetTest

[earlephilhower]

Seems okay, will be a noop once a new ::begin happens since the IP config is overwritten unconditionally there. Thx!