tuntap: add IFCAP_TXCSUM and IFCAP_TXCSUM_IPV6

[timo.voelker_fh-muenster.de]

Summary

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.

Test Plan

Run

ifconfig tap0 create
ifconfig tap0 txcsum txcsum6

and verify using

ifconfig tap0

that the offloading flags are actually set.

[timo.voelker_fh-muenster.de]

No functional changes. Just modified comments and code style.

[tuexen]

@timo.voelker_fh-muenster.de Is it intended that this patch now contains the changes from D51289?

[timo.voelker_fh-muenster.de]

Removed the changes that are in review D51289.

[timo.voelker_fh-muenster.de]

In D51291#1184658, @tuexen wrote:

@timo.voelker_fh-muenster.de Is it intended that this patch now contains the changes from D51289?

No it wasn't. Thanks for noting.

[timo.voelker_fh-muenster.de]

This patch was needed for D51688, but D51688 has changed and now uses the fact that the capabilities are added when a vnet_hdr_len > 0 is set, which obsoletes this patch.