[WIP] os_nodetype: impl with libuv

step towards #133
Astute reader will note that this block is the same as the old `mch_nodetype`. That's because BLK and DIR distinction is not made by `uv_guess_handle`. I'm going to see if it makes sense to modify libuv's `uv_guess_handle` to return handle types so that we can distinguish BLK and DIR there for unix.