* Define an interface for IOFS handling. Add no-op and temporary directory implementations.
* Move IOFS stuff out into separate file, add basic tmpfs implementation for linux only
* Switch between directory and tmpfs based on platform and config
* Respect FN_IOFS_OPTS
* Make directory iofs default on all platforms
* At least try to clean up a bit on failure
* Add backout if IOFS creation fails
* Add comment about iofs.Close
