When starting up a server with a storage engine that does not provide --directoryperdb support (eg. rocksExperiment, devnull, ...), the storage engine silently ignores --directoryperdb and does not adopt a different layout for the data directory.

To reproduce, build mongod with --rocks and run the follow JS Test:

buildscripts/smoke.py --storageEngine=devnull jstests/noPassthrough/directoryperdb.js