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_unix.go

@@ -0,0 +1,16 @@
+// +build !windows,!darwin
+
+package pidfile
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+func processExists(pid int) bool {
+	if _, err := os.Stat(filepath.Join("/proc", strconv.Itoa(pid))); err == nil {
+		return true
+	}
+	return false
+}