clearly picking one's nose isn't pathological and barbequing oneself or another is, but it seems more like a spectrum than a line. Mammals, at any rate, enjoy eating regenerable aspects of the body - breastfeeding, licking wounds, the whole amniotic process, golden showers, chewing off flaky skin parts, etc.
from the autophagy page on Wikipedia, I went to a link that said there is disagreement as to whether things like nailbiting amount to autocannibalism. Pathologically speaking, that is. If you want to be super literal, yes, eating skin would make you a cannibal while hair and nails would not (since it's not "flesh" in the normal sense).
…evil in the form of two rather appalling manifestations: a cannibal and an autophage. He notes that our attention is immediately captivated by one…while the other is largely overlooked. One possible explanation for this, he suggests, is that we have no place to put the notion of a person who consumes his or her own flesh; this is a form of being with the self that we cannot really conceive of.