Rather than having the if_vmove() code intermixed in the vnet_destroy()
function in vnet.c move it to if.c where it logically belongs and put
it under a VNET_SYSUNINIT() call.
To not change the current behaviour make sure it runs first thing
during teardown. In the future this will allow us more flexibility
on changing the order on when we want to get rid of interfaces.

Stop exporting if_vmove() and make it file static.

Reviewed by: gnn
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6438