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.
Pipe accept benchmarks have never been implemented, remove the code path.
Said code path also contained a bug: it tried to bind to the same pipe that is
bound to a few lines down.