os: use wait6 to avoid wait/kill race on netbsd

os: use wait6 to avoid wait/kill race on netbsd

Resend of CL 315281 which was partially reverted by CL 354249 after the
original CL was suspected to cause test failures as reported in #48789.
It seems that both wait4 and wait6 lead to that particular deadlock, so
let's use wait6. That way we at least don't hit #13987 on netbsd.

Updates #13987
For #48789
For #50138

Change-Id: Iadc4a771217b7e9e821502e89afa07036e0dcb6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/431855
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>