[horde] listFolders function for vfs file backend

[Liam Hoekenga]

Borrowed heavily from the FTP backend.... it seems to work ok - eg the 
"move / copy to" folder list in Gollem works now when using the VFS 
"file" driver:

     http://www.umich.edu/~liamr/horde/vfs-file-listFolders.diff

I've not put it through a lot of testing.

Liam