cfg80211/nl80211: add IBSS API

This adds IBSS API along with (preliminary) wext handlers.
The wext handlers can only do IBSS so you need to call them
from your own wext handlers if the mode is IBSS.

The nl80211 API requires
 * an SSID
 * a channel (frequency) for the case that a new IBSS
   has to be created

It optionally supports
 * a flag to fix the channel
 * a fixed BSSID

The cfg80211 code also takes care to leave the IBSS before
the netdev is set down. If wireless extensions are used, it
also caches values when the interface is down and instructs
the driver to join when the interface is set up.

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