Improve handling of ENOBUFS by not running in a tight loop forever.

Improve handling of ENOBUFS by not running in a tight loop forever.

This cherry-picks upstream git commit
0ee44ef384593ed0382d1ce6048d5a9c9440b45c.

Issue reported by landry@, which could be traced back to a buggy WiFi
driver that would constantly return ENOBUFS. This in turn lead
unwind(8) enter a tight loop through poll / sendto without making any
progress and burning 100% cpu.

OK sthen

I missed to commit this file when commiting to unbound.
Pointed out by tb, thanks!