etc/devd: recognize wireless network adapters

etc/devd: recognize wireless network adapters

Extend default devd(8) configuration to recognize wireless network
adapters, and automatically create child wlanX interfaces on attach.
Interfaces are created either according to rc.conf(5) or, in case
particular wireless device is not configured there, first unused
wlan is created for it.

Note: newly created wlan interface is not automatically brought up
and connected to any network. If such behaviour is desired, both
dhcpcd(8) and wpa_supplicant(8) (in interface matching mode) have to
be running (dhcpcd_enable and wpa_supplicant_enable in rc.conf(5)).

Discussed with: Sascha Wildner