Amend FindProcess to use pidfdFind, and make it return ErrProcessDone
if pidfdFind is used and the process is not found.
Since this is a change in API, introduce GODEBUG osfinderr=0 setting
to disable the feature.
Change-Id: I724c6f622f0c99f21a70b864cf7cf2b8836869ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/542699
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Than McIntosh <thanm@google.com>