mount,cmount,mount2: add --direct-io flag to force uncached access

This change adds the --direct-io flag to the mount. This means the
page cache is completely bypassed for reads and writes. No read-ahead
takes place. Shared mmap is disabled.

This is useful to accurately read files which may change length
frequently on the source.

cmd/mount2/node.go

@@ -158,6 +158,9 @@ func (n *Node) Open(ctx context.Context, flags uint32) (fh fusefs.FileHandle, fu
 	if entry := n.node.DirEntry(); entry != nil && entry.Size() < 0 {
 		fuseFlags |= fuse.FOPEN_DIRECT_IO
 	}
+	if n.fsys.opt.DirectIO {
+		fuseFlags |= fuse.FOPEN_DIRECT_IO
+	}
 	return newFileHandle(handle, n.fsys), fuseFlags, 0
 }