Move libworker_event_done_cb() from libworker.h to worker.h.

Move more prototypes around, and remove one that wasn't used.

Approved by: re (blanket)

Numerous fixes to make Unbound compile cleanly:

- cast through void * to silence alignment warnings (presumably false
positives resulting from poor API design)

- constify a few function arguments

- move prototypes for callbacks into a common header

- now that the prototypes are in scope, fix instances of function
definitions that don't match the prototype or what the caller
actually passes

- hide a conditionally unused global variable behind the same #ifdef
that controls its use

Approved by: re (blanket)