Massively improve error handling, check for EPOLLHUP & EPOLLERR etc.

- Make sure to listen for EPOLLRDHUP in watchers so users can be
  notified when a peer closes a connection.
- Handle EPOLLHUP and EPOLLERR gracefully.  Even handle spurious
  EPOLLHUP by means of an error counter.  If we receive too many errors
  we now restart the epoll fd and re-add all watchers.
- Return error from `epoll_ctl()` in `uev_watcher_stop()`
- Ignore any error from `uev_watcher_stop()` in `uev_watcher_set()`
- Move LIST_REMOVE() et al from io.c to core uev.c, should be
  done for timer and signal callbacks as well, of course.

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

io.c

@@ -62,11 +62,8 @@ int uev_io_init(uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int fd, int e
  */
 int uev_io_set(uev_t *w, int fd, int events)
 {
-	if (uev_io_stop(w))
-		return -1;
-
-	/* Remove from internal list */
-	LIST_REMOVE(w, link);
+	/* Ignore any errors, only to clean up anything lingering ... */
+	uev_io_stop(w);
 
 	return uev_io_init(w->ctx, w, (uev_cb_t *)w->cb, w->arg, fd, events);
 }
@@ -90,9 +87,7 @@ int uev_io_start(uev_t *w)
  */
 int uev_io_stop(uev_t *w)
 {
-	int status = uev_watcher_stop(w);
-
-	return status;
+	return uev_watcher_stop(w);
 }
 
 /**