Revert the netisr changes. Introduce a per-VNET flag to enable/disable
netisr prcessing on that VNET. Add functions to toggle the state.
Update consumers. Upon disabling netisr processing for a VNET drain
the netisr queue.

In if_attachdomain1() there is no need for trylock.
Also do not update the status before the initialisation has happened.

Reshuffle things a bit more in if_detach_internal(); always set the
interface down; set the if_afdata to NULL after clearing it.

In the vmove functions, as well as the if_ioctl, make sure the VNET
we are working on is not in the process of shutting down already.

Add a couple of __ununsed to vnet.c functions. Improve DDB printing
there to properly zero-prefix hex numbers to 8 digits.

Add an extra ifp argument to nd6_ifdetach() in order to be able to
walk the address list and stop DAD processing.

In nd6_timer() do not reset the callout before we are done with
handling everything.

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