unix: check UV_CLOSING flag in uv__write()
unix: check UV_CLOSING flag in uv__write()

uv__write() runs after the read callbacks have fired. Said callbacks may have
closed the handle, handle that graciously.

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