This permits the program to reliably know that when the Close method
returns, the descriptor has definitely been closed. This matters at
least for listeners.

Fixes #21856
Updates #7970

Change-Id: I1fd0cfd2333649e6e67c6ae956e19fdff3a35a83
Reviewed-on: https://go-review.googlesource.com/66150
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <joetsai@google.com>