Our fileops API is currently private.  We aren't planning on supplying
a cross-platform file API to applications that link to us.  If we did,
we'd probably whole-sale publish fileops, not just the dirent code.

By moving it to be private we can also change the call signature to
permit the buffer to be passed down through the call chain.  This is
very helpful when we are doing a recursive scan as we can reuse just
one buffer in all stack frames, reducing the impact the recursion has
on the stack frames in the data cache.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>