A tail of two shitties: This cow's rear end could hold the key to our future food supply, in a manure of speaking.Photo: ton.volzI never thought I'd see the day when shit -- the bodily kind -- would make headlines the way it is right now. When my book about managing manure, Holy Shit, came out recently, erstwhile friends grinned and remarked, "You've been shooting the bull all your life so, sure, why not write a book about it?" But this time what I'm writing is definitely not B.S. The current fertilizer crisis is real. Chemical fertilizer prices rise and …