[io] CompositeSource: added some funcs needed for resurrected sendfile() support.
[io] CompositeSource: added some funcs needed for resurrected sendfile() support.

christianparpart/x0,xzero/x0,christianparpart/x0,xzero/x0,christianparpart/x0,christianparpart/x0,christianparpart/x0,xzero/x0,xzero/x0,xzero/x0,christianparpart/x0,xzero/x0,christianparpart/x0