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_posix.go

@@ -0,0 +1,83 @@
+// 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.
+
+//go:build darwin || dragonfly || freebsd || linux || openbsd
+// +build darwin dragonfly freebsd linux openbsd
+
+package sysconf
+
+func sysconfPOSIX(name int) (int64, error) {
+	switch name {
+	case SC_ADVISORY_INFO:
+		return _POSIX_ADVISORY_INFO, nil
+	case SC_ASYNCHRONOUS_IO:
+		return _POSIX_ASYNCHRONOUS_IO, nil
+	case SC_BARRIERS:
+		return _POSIX_BARRIERS, nil
+	case SC_CLOCK_SELECTION:
+		return _POSIX_CLOCK_SELECTION, nil
+	case SC_CPUTIME:
+		return _POSIX_CPUTIME, nil
+	case SC_FSYNC:
+		return _POSIX_FSYNC, nil
+	case SC_IPV6:
+		return _POSIX_IPV6, nil
+	case SC_JOB_CONTROL:
+		return _POSIX_JOB_CONTROL, nil
+	case SC_MAPPED_FILES:
+		return _POSIX_MAPPED_FILES, nil
+	case SC_MEMLOCK:
+		return _POSIX_MEMLOCK, nil
+	case SC_MEMLOCK_RANGE:
+		return _POSIX_MEMLOCK_RANGE, nil
+	case SC_MONOTONIC_CLOCK:
+		return _POSIX_MONOTONIC_CLOCK, nil
+	case SC_MEMORY_PROTECTION:
+		return _POSIX_MEMORY_PROTECTION, nil
+	case SC_MESSAGE_PASSING:
+		return _POSIX_MESSAGE_PASSING, nil
+	case SC_PRIORITIZED_IO:
+		return _POSIX_PRIORITIZED_IO, nil
+	case SC_PRIORITY_SCHEDULING:
+		return _POSIX_PRIORITY_SCHEDULING, nil
+	case SC_RAW_SOCKETS:
+		return _POSIX_RAW_SOCKETS, nil
+	case SC_READER_WRITER_LOCKS:
+		return _POSIX_READER_WRITER_LOCKS, nil
+	case SC_REALTIME_SIGNALS:
+		return _POSIX_REALTIME_SIGNALS, nil
+	case SC_REGEXP:
+		return _POSIX_REGEXP, nil
+	case SC_SEMAPHORES:
+		return _POSIX_SEMAPHORES, nil
+	case SC_SHARED_MEMORY_OBJECTS:
+		return _POSIX_SHARED_MEMORY_OBJECTS, nil
+	case SC_SHELL:
+		return _POSIX_SHELL, nil
+	case SC_THREADS:
+		return _POSIX_THREADS, nil
+	case SC_TIMEOUTS:
+		return _POSIX_TIMEOUTS, nil
+	case SC_TIMERS:
+		return _POSIX_TIMERS, nil
+	case SC_VERSION:
+		return _POSIX_VERSION, nil
+
+	case SC_2_C_BIND:
+		return _POSIX2_C_BIND, nil
+	case SC_2_C_DEV:
+		return _POSIX2_C_DEV, nil
+	case SC_2_FORT_DEV:
+		return -1, nil
+	case SC_2_FORT_RUN:
+		return -1, nil
+	case SC_2_LOCALEDEF:
+		return _POSIX2_LOCALEDEF, nil
+	case SC_2_SW_DEV:
+		return _POSIX2_SW_DEV, nil
+	case SC_2_VERSION:
+		return _POSIX2_VERSION, nil
+	}
+	return -1, errInvalid
+}