windows: align uv_run code with unix
unix, windows: add uv_stop, stop running event loop
unix: make uv_stop work when embedded

* Make uv_stop() work when libuv is embedded in another event loop.

* Fix a small bug where loop->stop_flag was not reset when mode ==
  UV_RUN_ONCE or UV_RUN_NOWAIT. The next call to uv_run() would return
  immediately without doing any work.
windows: fix uv_stop in ONCE and NOWAIT modes

Same as the fix in 492efb9 but this time for uv-win.