API change -- Remove first ctx argument and add event mask to callback

There really does not exist a good reason for us sending a `uev_ctx_t *`
pointer as the first argument to event callbacks.  If the callback needs
to reference the context, it can find the ctx pointer in the `uev_t *w`.

This patch removes the first argument from all callbacks and also fixes
a slight bug in that the event loop never returned the actual event that
triggered the callback!

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

examples/joystick.c

@@ -40,7 +40,7 @@ struct js_event {
 	uint8_t  number;	/* axis/button number */
 } e;
 
-static void joystick_cb(uev_ctx_t *UNUSED(ctx), uev_t *w, void *UNUSED(arg), int UNUSED(events))
+static void joystick_cb(uev_t *w, void *UNUSED(arg), int UNUSED(events))
 {
 	if (read (w->fd, &e, sizeof(e)) < 0)
 		errx(errno, "Failed reading joystick event");