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

vfs/folder.go

@@ -141,6 +141,7 @@ type VirtualFolder struct {
 	QuotaFiles int `json:"quota_files"`
 }
 
+// GetFilesystem returns the filesystem for this folder
 func (v *VirtualFolder) GetFilesystem(connectionID string) (Fs, error) {
 	switch v.FsConfig.Provider {
 	case S3FilesystemProvider: