runtime: does not report duplicate errors in netpoll

Prevents storm of error messages if something goes wrong.
In the case of issue 5073 the epoll fd was closed by the test.
Update #5073.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/7966043

src/pkg/runtime/netpoll_epoll.c

@@ -57,6 +57,7 @@ runtime·netpollclose(int32 fd)
 G*
 runtime·netpoll(bool block)
 {
+	static int32 lasterr;
 	EpollEvent events[128], *ev;
 	int32 n, i, waitms, mode;
 	G *gp;
@@ -69,8 +70,10 @@ runtime·netpoll(bool block)
 retry:
 	n = runtime·epollwait(epfd, events, nelem(events), waitms);
 	if(n < 0) {
-		if(n != -EINTR)
-			runtime·printf("epollwait failed with %d\n", -n);
+		if(n != -EINTR && n != lasterr) {
+			lasterr = n;
+			runtime·printf("runtime: epollwait on fd %d failed with %d\n", epfd, -n);
+		}
 		goto retry;
 	}
 	gp = nil;

src/pkg/runtime/netpoll_kqueue.c

@@ -71,6 +71,7 @@ runtime·netpollclose(int32 fd)
 G*
 runtime·netpoll(bool block)
 {
+	static int32 lasterr;
 	Kevent events[64], *ev;
 	Timespec ts, *tp;
 	int32 n, i;
@@ -88,8 +89,10 @@ runtime·netpoll(bool block)
 retry:
 	n = runtime·kevent(kq, nil, 0, events, nelem(events), tp);
 	if(n < 0) {
-		if(n != -EINTR)
-			runtime·printf("kqueue failed with %d\n", -n);
+		if(n != -EINTR && n != lasterr) {
+			lasterr = n;
+			runtime·printf("runtime: kevent on fd %d failed with %d\n", kq, -n);
+		}
 		goto retry;
 	}
 	for(i = 0; i < n; i++) {