Fix VFS layer:

Fix VFS layer:
1. Finally work with cascaded modules with private data storage per module
2. Convert VFS API to macro calls to simplify cascading
3. Add quota support to VFS layer (prepare to NT quota support)

Patch by Stefan (metze) Metzemacher, with review of Jelmer and me
Tested in past few weeks. Documentation to new VFS API for third-party developers to follow
(This used to be commit 91984ef5caa2d13c5d52e1f535bd3bbbae1ec978)