Define a new method for probing vt(4) driver before attach it at early stage.

Sponsored by:	The FreeBSD Foundation
Create dataset for vt(4) drivers.

Sponsored by:	The FreeBSD Foundation
The tcp_log_addrs() uses th pointer, which points into the mbuf, thus we
can not free the mbuf before tcp_log_addrs().

Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix