lib: Maintain a representative event category inside internal state

lib: Maintain a representative event category inside internal state

More than one category can be represented by the internal state.  To give
consumers a unique but consistent category pointer, we allocate a category
structure (technically part of the internal state) and use it as the
representative for all the registrations.

Note: This commit only allocates and maintains the representatives.  It does
not make use of them.