Syntheses of 3-Substituted 1-Methyl-6-phenylpyrimido[5,4-e]-1,2,4-triazine-5,7(1H, 6H)-diones (6-Phenyl Analogs of Toxoflavin) and Their 4-Oxides, and Evaluation of Antimicrobial Activity of Toxoflavins and Their Analogs