Synthesis and Biological Activity of the Isomers and Analogs of (4E, 8E, 2S, 3R, 2'R)-N-2'-Hydroxyhexadecanoyl-9-methyl-4, 8-sphingadienine, the Ceramide Portion of the Fruiting-inducing Cerebroside in a Basidiomycete Schizophyllum commune