[RFC] Union mount readdir support in glibc

[Miklos Szeredi]

> > Actually, do we really need it other than to 0 and to current position
> > (i.e. full rewind and a no-op)?
> 
> Ever heard of the little function "telldir"?

Actually, telldir/seekdir is already broken for some filesystems (NFS
comes to mind).  POSIX was really crazy to require a working seekdir
implementation, and userspace should quickly start _not_ using it.

The more new filesystems it doesn't work, the better, IMHO.

Miklos