Add support for a new type of plugin which can be used to detect changes and
take some action whenever a subordinate entry is modified as a result of a
modify DN operation that targets an entry that has one or more children (i.e.,
a subtree move or subtree rename operation).  At present, subordinate modify DN
plugins are not allowed to change the contents of the entry as it is being
moved/renamed, but an appropriate API is in place if we decide to add this
functionality in the future.

This commit also includes a significant amount of cleanup for plugin result
code, including corrections to copy-and-paste errors in the javadoc
documentation.

OpenDS Issue Number:  752