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>

uev.h

@@ -57,6 +57,7 @@ struct uev;
 typedef struct {
 	int             running;
 	int             fd;	/* For epoll() */
+	uint32_t        errors;
 	LIST_HEAD(,uev) watchers;
 } uev_ctx_t;