Some progress, not much

* package.lisp (#:cl-guard-backend): New stuff to export.

* kqueue/guard-kqueue.lisp (kqueue-event, file-watch, directory-watch): New classes.
(scan): New function.
(event-names): New function, still unfinished.
(cl-guard-backend:add-watch, cl-guard-backend:remove-watch): Redesign, unfinished.
(cl-guard-backend:read-events): Redesign, unfinished.