Support running fuse-overlayfs without the open fds 0, 1 and 2

When running fuse-overlayfs with redirections `<&- >&- 2>&-`, the files don't show up at the mount point even though the command exits with a success.

To make it work, one has to run the command with `/dev/null 2>/dev/null`.

Reproduce the issue by running fuse-overlayfs with file descriptors 0, 1, and 2 closed, then compare it with the /dev/null redirection case. Trace startup and mount initialization to determine why the mount appears empty; done means files are visible at the mount point and the command succeeds with all three descriptors closed.

Written by the indexing model from the issue text.