In DNS Flag Day 2020, we started setting the DF (Don't Fragment socket
option on the UDP sockets. It turned out, that this code was incomplete
leading to dropping the outgoing UDP packets.
This has been now remedied, so it is possible to disable the
fragmentation on the UDP sockets again as the sending error is now
handled by sending back an empty response with TC (truncated) bit set.
This reverts commit 66eefac78c.