Fix stopAllExcept with WiFiClientSecure (#8136)

Fixes #8079

Because WiFiClientSecure inherits WiFiClient, and WiFiClientSecureCtx also
inherits WiFiClient, they both end up in the list of TCP connections that
are used for WiFiClient::stopAllExcept().  This would cause the underlying
SSL connection to be closed whenever you attempted to
stopAllExcept(WiFiClientSecure)

Fix by adding a "_owned"(by) pointer in the WiFiClient object which points to
nullptr (default case) or to the associated lower-layer connection.
When stopping all connections except one, only look at the lowermost
connections.

libraries/ESP8266WiFi/src/WiFiClient.h

@@ -144,6 +144,7 @@ protected:
   void _err(int8_t err);
 
   ClientContext* _client;
+  WiFiClient* _owned;
   static uint16_t _localPort;
 };