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/zsysconf_values_freebsd_arm.go

@@ -0,0 +1,12 @@
+// Code generated by cmd/cgo -godefs; DO NOT EDIT.
+// cgo -godefs sysconf_values_freebsd.go
+
+//go:build freebsd && arm
+// +build freebsd,arm
+
+package sysconf
+
+const (
+	_LONG_MAX = 0x7fffffff
+	_SHRT_MAX = 0x7fff
+)