Add support for bridging iwn(4) based wlan(4)

Documented in iwn(4), "Only one virtual interface may be configured at any
time." However, netgraph with a cloned MAC address is able to communicate
over an ng_eiface attached to an ng_bridge linked to the wlan(4) interface.
While here, introduce syntax to specify the MAC address is to be cloned if
the named interface begins with equals [=].