New function to check string

If there are only alphanumeric chars (including underscore) return 1
else return 0

esp-link/cgiwifi.h

@@ -19,6 +19,7 @@ void configWifiIP();
 void wifiInit(void);
 void wifiAddStateChangeCb(WifiStateChangeCb cb);
 void wifiStartMDNS(struct ip_addr);
+int checkString(char *str);
 
 extern uint8_t wifiState;
 extern bool mdns_started;