Add capability flag for IBSS and get_capabiliy modes

Add a driver capability flag for drivers which support IBSS mode and
set it for nl80211 drivers which have set the NL80211_IFTYPE_ADHOC.

Add a new option "modes" to "get_capability" which will return
"AP" and "IBSS" if the corresponding capability flags are set.

Change-Id: I7991ae49b6ac6bc154d4edb0b01af774f4e8522c