[2020] unbound should also set max-udp-size

[chantra]

It is my understanding that edns-buffer-size will only set a EDNS buffer size from recursive to authoritative, but the recursive to stub answer may be greater than that.

Similarly to PowerDNS's udp-truncation-threshold, and Bind and Knot's max-udp-size/max-udp-payload, Unbound has max-udp-size.

Should this be recommended to be set to 1232 too in order to cap recursive to stub response size? cc @ralphdolmans