Wifi: Add IBSS supported method

Add method isIbssSupported() to WifiManager and related classes to query wether
the driver supports IBSS mode. The query is answered by wpa_supplicants new
command "get_capability modes".

It will only return a correct result after WIFI_STATE_ENABLED has been
reached.

Change-Id: I93aa2fdff94cd897c91ec3beb7e58940d1de7b72