nl80211: report BSS status

When connected to a BSS, or joined to an IBSS, we'll want
to know in userspace without using wireless extensions, so
report the BSS status in the BSS list. Userspace can query
the BSS list, display all the information and retrieve the
station information as well.

For example (from hwsim):

$ iw dev wlan1 scan dump
BSS 02:00:00:00:00:00 (on wlan1) -- associated
	freq: 2462
	beacon interval: 100
	capability: ESS ShortSlotTime (0x0401)
	signal: -50.00 dBm
	SSID: j
	Supported rates: 1.0* 2.0* 5.5* 11.0* 6.0 9.0 12.0 18.0
	DS Paramater set: channel 11
	ERP: <no flags>
	Extended supported rates: 24.0 36.0 48.0 54.0

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>