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/mkdirall.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package pidfile
+
+import "os"
+
+// MkdirAll creates a directory named path along with any necessary parents,
+// with permission specified by attribute perm for all dir created.
+func MkdirAll(path string, perm os.FileMode) error {
+	return os.MkdirAll(path, perm)
+}