Make the in-kernel logic for the SIOCSIFVNET, SIOCSIFRVNET ioctls

Make the in-kernel logic for the SIOCSIFVNET, SIOCSIFRVNET ioctls
(ifconfig ifN (-)vnet <jname|jid>) work correctly.

Move vi_if_move to if.c and split it up into two functions(*),
one for each ioctl.

In the reclaim case, correctly set the vnet before calling if_vmove.

Instead of silently allowing a move of an interface from the current
vnet to the current vnet, return an error. (*)

There is some duplicate interface name checking before actually moving
the interface between network stacks without locking and thus race
prone. Ideally if_vmove will correctly and automagically handle these
in the future.

Suggested by:	rwatson (*)
Approved by:	re (kib)