(fs) Provide ability to flush all modified buffered data to a permanent storage device

This issue follows from a change in undocumented behavior introduced by the JDK 9 fix of JDK-8066915. Previously in order to sync data to a directory, a FileChannel was opened in READ mode and then FileChannel.force() was invoked to flush modified in-state buffers and possibly metadata to the storage device. This behavior was however undocumented and not intended to be supported as for example a read() on the same directory FileChannel is unsupported and would provoke an exception.

The intent of this issue is to provide an alternate means to effect the desired behavior of force() within the bounds of the specification. This might call for new field definitions or new methods.