Let users control whether unix listener socket file is unlinked on close.

Fixes #13877.

Change-Id: I9d1cb47e31418d655f164d15c67e188656a67d1c
Reviewed-on: https://go-review.googlesource.com/32099
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>