This changes the prototype of uv_run() from:
int uv_run(uv_loop_t* loop);
To:
int uv_run(uv_loop_t* loop, uv_run_mode mode);
Where `mode` is UV_RUN_DEFAULT, UV_RUN_ONCE or UV_RUN_NOWAIT.
Fixes#683.
Bert Belder informs me the current approach where a request is immediately
cancelled, is impossible to implement on Windows.
Rework the API to always invoke the "done" callback with an UV_ECANCELED error
code.
