Change to using gopsutil for cross-OS process ops

Instead of trying to write out own code to do basic process
operations (e.g. checking if a PID is still running in a multi-OS
friendly manner), use shirou/gopsutil, a multi-platform library
that should abstract all the complexity away. Unlike our previous
approach on Windows, this one should actually work.

Signed-off-by: Matt Heon <mheon@redhat.com>

vendor/github.com/tklauser/go-sysconf/sysconf_unsupported.go

@@ -0,0 +1,17 @@
+// Copyright 2021 Tobias Klauser. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !darwin && !dragonfly && !freebsd && !linux && !netbsd && !openbsd && !solaris
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
+
+package sysconf
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func sysconf(name int) (int64, error) {
+	return -1, fmt.Errorf("unsupported on %s", runtime.GOOS)
+}