rofiles-fuse: Rework to be based on nlink
rofiles-fuse: Rework to be based on nlink

Programs like `useradd` try to `open(/etc/passwd, O_RDWR)` to append,
which didn't work with rofiles-fuse.  Thinking about this, I realized
that there's a simpler algorithm for "can we write to this file" which
is "does it have a hardlink count <= 1"?

Switching to this both drops complexity (we no longer need to keep a
hash table of files we created), and also lets useradd work.

Closes: #462
Approved by: jlebon

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree