mount: allow files of unkown size to be read properly

Before this change, files of unknown size (eg Google Docs) would
appear in file listings with 0 size and would only allow 0 bytes to be
read.

This change sets the direct_io flag in the FUSE return which bypasses
the cache for these files.  This means that they can be read properly.

This is compatible with some, but not all applications.

cmd/cmount/fs.go

@@ -299,6 +299,9 @@ func (fsys *FS) Open(path string, flags int) (errc int, fh uint64) {
 		return translateError(err), fhUnset
 	}
 
+	// FIXME add support for unknown length files setting direct_io
+	// See: https://github.com/billziss-gh/cgofuse/issues/38
+
 	return 0, fsys.openHandle(handle)
 }