Add IFCAP_TXCSUM and IFCAP_TXCSUM_IPV6 to if_capabilities of tun/tap interfaces. If enabled, csum flags are translated into virtio-net header flags. If the virtio-net header is not available, an error gets returned when reading from the character device.
This will be used to add tx checksum offload functionality for a bhyve guest.

Run

ifconfig tap0 create
ifconfig tap0 txcsum txcsum6

and verify using

ifconfig tap0

that the offloading flags are actually set.