Add a knob to allow zero UDP checksums for UDP/IPv6 traffic on the given UDP port.

This will be used by some upcoming changes to if_vxlan(4). RFC 7348 (VXLAN)
says that the UDP checksum "SHOULD be transmitted as zero. When a packet is
received with a UDP checksum of zero, it MUST be accepted for decapsulation."
But the original IPv6 RFCs did not allow zero UDP checksum. RFC 6935 attempts
to resolve this.

Reviewed by: kib@
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D25873