Fuse mount: make auto_unmount compatible with suid/dev mount options (#762)

Fuse mount: make auto_unmount compatible with suid/dev mount options (#762)

* Fuse mount: make auto_unmount compatible with suid/dev mount options

> When you run as root, fuse normally does not call fusermount but uses
> the mount system call directly. When you specify auto_unmount, it goes
> through fusermount instead. However, fusermount is a setuid binary that
> is normally called by regular users, so it cannot in general accept suid
> or dev options.

In this patch, we split up how fuse mounts as root when `auto_unmount`
is specified.

First, we mount using system calls directly, then we reach out to
fusermount to set up auto_unmount only (with no actual mounting done in
fusermount).

Fixes: #148