Add capability flag for IBSS and add get_capability modes

Add capability flag for IBSS and add get_capability 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.

The idea is that this can be used for UIs to find out if the driver
supports IBSS mode.

Signed-hostap: Bruno Randolf <br1@einfach.org>