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/lufia/plan9stats/opts.go

@@ -0,0 +1,21 @@
+package stats
+
+type Config struct {
+	rootdir string
+}
+
+type Option func(*Config)
+
+func newConfig(opts ...Option) *Config {
+	var cfg Config
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+	return &cfg
+}
+
+func WithRootDir(dir string) Option {
+	return func(cfg *Config) {
+		cfg.rootdir = dir
+	}
+}