Virtualise the netisr registration in order to do a per-vnet
de-registration to prevent further packets for a specific protocol
(IP, ARP, IPv6) to come up from ether_demux().
We currently have not better "plug-and-play" hook in place at that
level but should think about that in some distant future (e.g.,
to one day be able to load ip, or ipv6).

Note: this commit will be reverted soon. It turns out that while
the idea is good, and basically works, it can possibly lead to
deadlocks.

Sponsored by: The FreeBSD Foundation