internal/poll: don't return from Close until descriptor is closed
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>