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.go

@@ -0,0 +1,21 @@
+// Copyright 2018 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.
+
+// Package sysconf implements the sysconf(3) function and provides the
+// associated SC_* constants to query system configuration values.
+package sysconf
+
+import "errors"
+
+//go:generate go run mksysconf.go
+
+var errInvalid = errors.New("invalid parameter value")
+
+// Sysconf returns the value of a sysconf(3) runtime system parameter.
+// The name parameter should be a SC_* constant define in this package. The
+// implementation is GOOS-specific and certain SC_* constants might not be
+// defined for all GOOSes.
+func Sysconf(name int) (int64, error) {
+	return sysconf(name)
+}