Handle uh_sum == 0x0000 correctly. Before, UDP packet checksums were broken by NAT/RDR when unset by the sender. Fixes ntpdate behind NAT.
Handle uh_sum == 0x0000 correctly. Before, UDP packet checksums were
broken by NAT/RDR when unset by the sender. Fixes ntpdate behind NAT.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars