Update the base DN registration process to address a number of issues:

- It fixes problems with base DN registration when attempting to use nested
  backends.

- It changes the terminology that is in use.  Previously, the term "suffix" was
  used everywhere, even when it wasn't really correct.  Now "naming context" is
  used when it's talking about a top-level base DN, and "base DN" is used for
  any base DN regardless of whether it's a naming context.

- It adds a new ds-private-naming-contexts operational attribute to the root
  DSE to list the private naming contexts defined in the server.

- It updates the backend API to get rid of the abstract supportsControl and
  supportsFeature methods and replace them with a default concrete
  implementation.


OpenDS Issue Numbers:  546, 750