Amend FindProcess to use pidfdFind, and make it return a special
Process with Pid of pidDone (-2) if the process is not found.
Amend Wait and Signal to return ErrProcessDone if pid == pidDone.
The alternative to the above would be to make FindProcess return
ErrProcessDone, but this is unexpected and incompatible API change,
as discussed in #65866 and #51246.
For #62654.
Rework of CL 542699 (which got reverted in CL 566476).
Change-Id: Ifb4cd3ad1433152fd72ee685d0b85d20377f8723
Reviewed-on: https://go-review.googlesource.com/c/go/+/570681
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>