regedit: make all hives descend from a root node

regedit: make all hives descend from a root node

This helps simplify cleanup, since each node's talloc context is the
parent node, and freeing the root node will destroy the entire tree
without any extra utility functions.

It also wouldn't be hard to extend this later on to support browsing
multiple registries at the same time.

Signed-off-by: Chris Davis <cd.rattan@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>