Option for files but not directories to persist

[jcnelson]

There's a good use-case for a long-lived process making a directory, but populating it with short-lived processes (i.e. a daemon invoking one-off helper programs running as subprocesses). There should be a way to tell runfs to allow files to persist within a directory--perhaps through an extended attribute on the directory (e.g. "user.runfs.persistent_files=1" or something).