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.
* the callback gets called only once on error, not repeatedly...
* ...unless the error reason changes from e.g. UV_ENOENT to UV_EACCES
* the callback receives pointers to uv_statbuf_t objects so it can inspect what
changed
