Add counter for Tx errors returned from if_transmit.

Submitted-by:	Boris Misenov <Boris.Misenov@oktetlabs.ru>
Sponsored by:	Solarflare Communications, Inc.
When exporting ifnet via sysctl, add ifqueue(9) drop count to the
ifi_oqdrops. This is a temporary workaround until ifqueue(9) vanishes.

While here, remove the pointless ifi_vhid assignment. It has
sense only when we are exporting ifaddrs, not ifnets.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.