rc.d/netif: Rework wlan interface configuration

rc.d/netif: Rework wlan interface configuration

* wlan interfaces are more similar to clonable interfaces rather than
  vlan interfaces, so move the relevant code from childif_create() to
  network.subr as wlan_up(), which will be called to create the wlan
  interfaces before setting up them.

  Now, list_net_interfaces() shouldn't list "net.wlan.devices".

* Add the wlan_down() function in network.subr to destroy the wlan
  devices.

* Meanwhile, fix childif_destroy() to properly destroy the vlan
  interfaces created by childif_create().  Also tweak the order of this
  function in netif_stop().

* The 2-second delay in netif_start() doesn't apply anymore, since the
  DHCP and WPA configurations are now handled in ifconfig_up() called by
  ifn_start().

This patch is loosely based on FreeBSD's code.  Thanks to them.