Add overloads for moving/copying files that flushes caches

[JeremyKuhne]

For critical data it might be good to have the ability to flush pending operations to disk. We allow you to do this with FileStream, but not copy/move/rename.

Rename might be awkward as Windows doesn't directly support disabling the cache, other APIs in Windows have flags that let you do this.

In Unix flushing the cache is done with fsync.

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