etc/network.subr: enhance wlan initialization

etc/network.subr: enhance wlan initialization

Introduce new functions wlan_get_unused() and wlan_is_parent(),
and extend wlan_up() to utilize them.

With this change, devices configured the traditional way in rc.conf(5)
are created first, and then all other wireless interfaces are assigned
to next available wlan<index> devices automatically.

This allows either to have wlan configured manually in rc.conf(5) via
"wlans_<iface>" or to remove the static assignment and let devices be
auto-created. With dhcpcd_enable and wpa_supplicant_enable in rc.conf,
no further configuration is then needed to get network connectivity.

Furthermore, this will allow to enhance devd(8) default configuration,
so that also hot-pluggable wireless interfaces are automatically detected
and brought up (ie. assigned to wlan<index> devices).

Reviewed by: Aaron LI