Put structure into anno(1) and its man page.

The ``newly'' added features (-list, -delete, ...) were simply added
on top, but without rethinking and adjusting anno(1)'s structure. (I
agree, however, that it was the right decision to add these features
to anno(1) instead of writing another tool.) I've reworked anno now to
give it a clear structure. And I've reworked its man page to propagate
this structure.