bindfs: Let `mount` find `bindfs` mount helper

When mount is used with mount-type "fuse.bindfs", it cannot find the
mount helper.

If mount can't find `mount.fuse.bindfs`, it executes the `mount.fuse`
mount helper and passes `fuse.bindfs` as argument. Then `mount.fuse`
tries to execute `bindfs` on its own, but it is not found in the PATH.

By creating a `mount.fuse.bindfs` link to the `bindfs` executable, this
problem is avoided because `mount` will just execute the
`mount.fuse.bindfs` mount helper without `mount.fuse` in the middle.

pkgs/tools/filesystems/bindfs/default.nix

···
   dontStrip = true;
 
   buildInputs = [ fuse pkgconfig ];
+  postFixup = ''
+    ln -s $out/bin/bindfs $out/bin/mount.fuse.bindfs
+  '';
 
   meta = {
     description = "A FUSE filesystem for mounting a directory to another location";