Consider processes attaching/detaching from tun(4) devices as being link
state changes.  This change modifies tunopen and tunclose to call the
if_link_state_change() function.  Among other things, this will result in
devd(8) receiving events from devctl(4) for linkup/link down.  This allows
us to do several useful things, including initializing tunnel parameters
and adding routes.

Discussed on:	freebsd-net