Guard against sending the handle over the UNIX domain socket twice
when the first sendmsg() didn't write all bytes.
The changes to src/win partially undo changes made earlier this year,
see the referenced pull request for details.
Libuv never made promises about the value of `req->send_handle` at
different points in time so this should be a safe, non-breaking change.
No tests because this particular condition is hard to hit reliably
across platforms. I spent a lot of time trying to write one but it
turned out hideously complex, and worse, flaky.
Fixes: https://github.com/libuv/libuv/issues/2086
PR-URL: https://github.com/libuv/libuv/pull/2097
Refs: https://github.com/libuv/libuv/pull/1843
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>