This change reverts CL 18814 which is a workaroud for older DragonFly
BSD kernels, and fixes #13945 and #13947 in a more general way the
same as other platforms except NetBSD.

This is a followup to CL 29491.

Updates #16329.

Change-Id: I771670bc672c827f2b3dbc7fd7417c49897cb991
Reviewed-on: https://go-review.googlesource.com/29971
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>