tailscale: Add tailssh server

experimental/libbox/ssh_shell.go

@@ -0,0 +1,9 @@
+package libbox
+
+type ShellSession interface {
+	MasterFD() int32
+	Resize(rows int32, cols int32) error
+	Signal(signal int32) error
+	WaitExit() (int32, error)
+	Close() error
+}