Changes to node, zone-contents + IXFR adding node

- node_new() sets flags to node.
- fixed node_set_previous() - properly connects the node to
  the chain of nodes.
- Modified calls to node_new() in all parts of the program.
- zone_contents_add_node() sets flags to created parents,
  checks if created nodes are not wildcard children,
  properly sets the last node's parent.
- Fixed zone_contents_add_nsec3_node() (parent setting).
- zone_contents_get_previous() + find/get_previous_nsec3() added.

- Adding new node during IXFR zone update implemented.

refs #1034 @2h