Make pathname resolve local adapter and local path.

This allow moving up and down in the hierarchy, even over aggregation
points. This also allow interaction between different filesystems (not
only adapters).
Add DelegatorAdapter.
Add RootAdapter, the adapter that should be the first in a filesystem.
Add DelegatorAdapter::selectDelegate().
Rework and clean up LocalAdapter.
Continue reworking and update the local adapter.
Continue reworking on stream implementation.
Reenable cleanup on test.