Synthesis and Inhibitory Effect on Platelet Aggregation and Antihypertensive Activity of 1-Hydroxy-2,5-dihydro-1H-imidazole-2-carboxylic Acid 3-Oxides, 1,3-Dihydroxyimidazolidine-2-carboxylic Acids, and 1,4-Dihydroxy-2,3-piperazinedio