Add pidfile support

Copy a simple implementation from the Moby project, since importing
their package would pull in too many dependencies.

Fixes #320

internal/pidfile/pidfile_darwin.go

@@ -0,0 +1,14 @@
+// +build darwin
+
+package pidfile
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func processExists(pid int) bool {
+	// OS X does not have a proc filesystem.
+	// Use kill -0 pid to judge if the process exists.
+	err := unix.Kill(pid, 0)
+	return err == nil
+}