Reactions of (PiPr3)2OsH6 Involving Addition of Protons and Removal of Electrons. Characterization of (PiPr3)2Os(NCMe)xHyz+ (x = 0, 2, 3; y = 1, 2, 3, 4, 7; z = 1, 2), Including Dicationic .eta.2-H2 Complexes