a different BSS, because that keeps drivers like atw from detecting
ad hoc beacons with the same SSID but different BSSID for the
purpose of IBSS merges.
This should be revisited: maybe drivers should check for beacons
before handing packets up with ieee80211_input(). However, this
restores the atw(4) to working like it did with the old 802.11
layer, and it *is* nice to re-use the 802.11 layer's packet-decode
& -dispatch code (i.e., the ic_recv_mgmt callback).
38a484b2d0