Copy logic from preflight necessary to iterate all scripts in the postfl...

I like having the ability to throw a script into the preflight.d or postflight.d folder when necessary. The preflight script already provided support for running everything in the preflight.d folder, but the postflight script was lacking this functionality.

I copied the functionality over to the postflight script and removed any functions that were specific to the preflight script. I've tested this change and it works as expected. Let me know if you require any other changes to this. (Or if this is functionality you do not want, that is fine as well).