Provide appropriate implementations of Tc{get,set}pgrp and export
these for use in the TestForeground* tests in exec_unix_test.go.
This avoids calling ioctl via syscall.Syscall on BSDs.
Fixes #59667
Updates #63900
Change-Id: Ice4dcedae1f0931c026bddf33043d3864a52d44e
Reviewed-on: https://go-review.googlesource.com/c/go/+/572155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>