sftpd: add support for excluding virtual folders from user quota limit

Fixes #110

vfs/vfs.go

@@ -52,6 +52,8 @@ type Fs interface {
 type VirtualFolder struct {
 	VirtualPath string `json:"virtual_path"`
 	MappedPath  string `json:"mapped_path"`
+	// This folder will be excluded from user quota
+	ExcludeFromQuota bool `json:"exclude_from_quota"`
 }
 
 // IsDirectory checks if a path exists and is a directory