branches: 1.2.2; 1.2.4; 1.2.18;
Move tun.c into the module's own directory, since it is specific to the
module subsystem.

Add a dummy "tun" module, whose only job is to trigger an autoload of
required module "if_tun". This allows access to /dev/tunN to autload
the require interface module.

XXX There's might be a better place/name for net/tun.c