add support for serving Google Cloud Storage over SFTP/SCP

Each user can be mapped with a Google Cloud Storage bucket or a bucket
virtual folder

vfs/fileinfo_unix.go

@@ -0,0 +1,28 @@
+// +build !windows
+
+package vfs
+
+import "syscall"
+
+import "os"
+
+var (
+	defaultUID, defaultGID int
+)
+
+func init() {
+	defaultUID = os.Getuid()
+	defaultGID = os.Getuid()
+	if defaultUID < 0 {
+		defaultUID = 65534
+	}
+	if defaultGID < 0 {
+		defaultGID = 65534
+	}
+}
+
+func (fi FileInfo) getFileInfoSys() interface{} {
+	return &syscall.Stat_t{
+		Uid: uint32(defaultUID),
+		Gid: uint32(defaultGID)}
+}