Follow-up to 2f9cce9, handle I/O watchers similar to timer+signal

This patch is a follow-up to 2f9cce9, and a complete refactor of
fe8f3f1 which first introduced the EPOLLERR and EPOLLHUP handling.
Then mainly for managing errors and hangups on stream sockets.

Now, instead of trying to reopen the epoll() fd, and breaking out
hard from the event loop on catastrophic error,  we now delegate
I/O watcher EPOLLERR and EPOLLHUP to the watcher callback, just
like other watchers.

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

src/private.h

@@ -44,7 +44,6 @@ typedef enum {
 typedef struct {
 	int             running;
 	int             fd;     /* For epoll() */
-	uint32_t        errors;
 	LIST_HEAD(,uev) watchers;
 	uint32_t        workaround; /* For workarounds, e.g. redirected stdin */
 } uev_ctx_t;