driver.h contains the definitions needed in driver wrapper
implementations (driver_*.c) and driver_i.h contains the definitions
that are used in core hostapd code to interact with the driver wrappers.
Changed accounting_sta_start() to call accounting_sta_get_id()
internally in accounting.c so that external callers do not need to do
anything to allocate unique accounting id. When starting a new session,
a unique identifier is needed anyway, so no need to keep these
operations separate.
