unix, windows: rename uv_run2 to uv_run

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.