Windows: manually check if we can bind on the configured port/ports

Windows allows the coexistence of three types of sockets on the same
transport-layer service port, for example, 127.0.0.1:8080, [::1]:8080
and [::ffff:0.0.0.0]:8080

Go don't properly handles this, so we use a ugly hack

Fixes #350

ftpd/ftpd.go

@@ -211,6 +211,7 @@ func (c *Configuration) Initialize(configDir string) error {
 		go func(s *Server) {
 			ftpServer := ftpserver.NewFtpServer(s)
 			logger.Info(logSender, "", "starting FTP serving, binding: %v", s.binding.GetAddress())
+			utils.CheckTCP4Port(s.binding.Port)
 			exitChannel <- ftpServer.ListenAndServe()
 		}(server)