update many dirnode interfaces to accept dict-of-nodes instead of dict-of-caps

update many dirnode interfaces to accept dict-of-nodes instead of dict-of-caps

interfaces.py: define INodeMaker, document argument values, change
               create_new_mutable_directory() to take dict-of-nodes. Change
               dirnode.set_nodes() and dirnode.create_subdirectory() too.
nodemaker.py: use INodeMaker, update create_new_mutable_directory()
client.py: have create_dirnode() delegate initial_children= to nodemaker
dirnode.py (Adder): take dict-of-nodes instead of list-of-nodes, which
                    updates set_nodes() and create_subdirectory()
web/common.py (convert_initial_children_json): create dict-of-nodes
web/directory.py: same
web/unlinked.py: same
test_dirnode.py: update tests to match