Allow for access to file open `FILE_SHARE_*` flags

[childish-sambino]

Similar to what’s described in this comment for WinFSP: winfsp/winfsp#116 (comment)

We’re interested in determining whether a file was requested to be opened exclusively by an application, as this would enable us to handle such requests appropriately. I believe having access to the underlying FILE_SHARE_* flags might provide the necessary context for this, though I’m not certain.

[billziss-gh]

The FILE_SHARE_* flags are available to native WinFsp file systems, but not FUSE file systems. Since FUSE originated in Linux, it does not know about the FILE_SHARE_* flags. Making them available to cgofuse would mean that the WinFsp-FUSE API would have to be extended (in a backwards compatible manner) to support them.