Crystal and Molecular Structures of the Antiviral Acyclonucleoside 9-[4-Hydroxy-3-(hydroxymethyl)butyl]guanine (BRL 39123, Penciclovir) and its Prodrug 9-[4-Acetoxy-3-(acetoxymethyl)butyl]-2-aminopurine (BRL 42810, Famciclovir)