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_solaris.go

@@ -0,0 +1,14 @@
+// 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.
+
+package sysconf
+
+import "golang.org/x/sys/unix"
+
+func sysconf(name int) (int64, error) {
+	if name < 0 {
+		return -1, errInvalid
+	}
+	return unix.Sysconf(name)
+}