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 <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: tklauser

src/os/wait_unimp.go

@@ -6,7 +6,7 @@
 // waitid/wait6. netbsd implements wait6, but that is causing test
 // failures, see issue #48789.
 
-//go:build aix || darwin || (js && wasm) || netbsd || openbsd || solaris
+//go:build aix || darwin || (js && wasm) || openbsd || solaris
 
 package os
 

src/os/wait_wait6.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build dragonfly || freebsd
+//go:build dragonfly || freebsd || netbsd
 
 package os