Add SME support (separate authentication and association)

Add SME support (separate authentication and association)

This can be used, e.g., with mac80211-based Linux drivers with
nl80211. This allows over-the-air FT protocol to be used (IEEE
802.11r).

Since the nl80211 interface needed for this is very recent (added
today  into wireless-testing.git), driver_nl80211.c has backwards
compatibility code that uses WEXT for association if the kernel does
not support the new commands. This compatibility code can be
disabled by defining NO_WEXT_COMPAT. That code will also be removed
at  some point to clean up driver_nl80211.c.