if_wg: export tx_bytes, rx_bytes, and last_handshake

[kevans]

Summary

The names are self-explanatory; these are currently only used by the
wg(8) tool, but they are handy data points to have.

MFC after: 3 days
Discussed with: decke

[markj]

sys/dev/if_wg/module/module.c

487

Why is this conditional?

[kevans]

sys/dev/if_wg/module/module.c

487

My working thought process was to avoid the nvpair allocation since the wg(8) side will assume '0' if these are omitted -- their absence is insignificant.