unix: make uv_stop work when embedded
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.

urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso