os: make use of pidfd on linux

Use Process.handle field to store pidfd, and make use of it. Only use
pidfd functionality if all the needed syscalls are available.

1. Add/use pidfdWorks, which checks that all needed pidfd-related
   functionality works.

2. os.StartProcess: obtain the pidfd from the kernel, if possible, using
   the functionality added by CL 520266. Note we could not modify
   syscall.StartProcess to return pidfd directly because it is a public
   API and its callers do not expect it, so we have to use ensurePidfd
   and getPidfd.

3. (*Process).Kill: use pidfdSendSignal, if available and the pidfd is
   known. Otherwise, fall back to the old implementation.

4. (*Process).Wait: use pidfdWait, if available, otherwise fall back to
   using waitid/wait4. This is more complicated than expected due to
   struct siginfo_t idiosyncrasy.

NOTE pidfdSendSignal and pidfdWait are used without a race workaround
(blockUntilWaitable and sigMu, added by CL 23967) because with pidfd,
PID recycle issue doesn't exist (IOW, pidfd, unlike PID, is guaranteed
to refer to one particular process) and thus the race doesn't exist
either.

Rework of CL 528438 (reverted in CL 566477 because of #65857).

For #62654.
Updates #13987.

Change-Id: If5ef8920bd8619dc428b6282ffe4fb8c258ca224
Reviewed-on: https://go-review.googlesource.com/c/go/+/570036
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

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