Better handling of wifi disconnect (#231)

When network interface is down, some nasty things happen, for instance tcp_connect returns without ever calling error callback.
This change adds some workarounds for that: before doing a tcp connect and DNS resolve we check if there is a route available.
Also added a listener for wifi events which stops (aborts) all the WiFiClients and WiFiUDPs when wifi is disconnected. This should
help libraries detect disconnect properly.

libraries/ESP8266WiFi/src/include/slist.h

@@ -0,0 +1,38 @@
+#ifndef SLIST_H
+#define SLIST_H
+
+template<typename T>
+class SList {
+public:
+  SList() : _next(0) { }
+
+protected:
+
+  static void _add(T* self) {
+    T* tmp = _s_first;
+    _s_first = self;
+    self->_next = tmp;
+  }
+
+  static void _remove(T* self) {
+    if (_s_first == self) {
+      _s_first = self->_next;
+      self->_next = 0;
+      return;
+    }
+
+    for (T* prev = _s_first; prev->_next; _s_first = _s_first->_next) {
+      if (prev->_next == self) {
+        prev->_next = self->_next;
+        self->_next = 0;
+        return;
+      }
+    }
+  }
+
+  static T* _s_first;
+  T* _next;
+};
+
+
+#endif //SLIST_H