MINOR: pollers: add a new flag to indicate pollers reporting ERR & HUP

In practice it's all pollers except select(). It turns out that we're
keeping some legacy code only for select and enforcing it on all
pollers, let's offer the pollers the ability to declare that they
do not need that.
diff --git a/src/ev_epoll.c b/src/ev_epoll.c
index dc156e5..8b2c2ee 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -360,7 +360,7 @@
 
 	p->name = "epoll";
 	p->pref = 300;
-	p->flags = 0;
+	p->flags = HAP_POLL_F_ERRHUP; // note: RDHUP might be dynamically added
 	p->private = NULL;
 
 	p->clo  = __fd_clo;