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

sftpd/server.go

@@ -232,6 +232,7 @@ func (c *Configuration) Initialize(configDir string) error {
 
 		go func(binding Binding) {
 			addr := binding.GetAddress()
+			utils.CheckTCP4Port(binding.Port)
 			listener, err := net.Listen("tcp", addr)
 			if err != nil {
 				logger.Warn(logSender, "", "error starting listener on address %v: %v", addr, err)