unix: clear UV_STREAM_SHUTTING after shutdown()
unix: clear UV_STREAM_SHUTTING after shutdown()

Fix a state machine buglet where the UV_STREAM_SHUTTING flag didn't get
cleared.

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