Ask driver to report only new scan results if requested

Ask driver to report only new scan results if requested

If the BSS table within wpa_supplicant is flushed, request the driver to
flush its own scan result table during the next scan. This can avoid
unexpected old BSS entries showing up after BSS_FLUSH or FLUSH command
in cases where the driver may maintain its internal cache of scan
results (e.g., cfg80211 BSS table persists at least for 15 seconds).

In addition to doing this automatically on BSS_FLUSH/FLUSH, a new SCAN
command argument, only_new=1, can be used to request a manual scan
request to do same. Though, it should be noted that this maintains the
BSS table within wpa_supplicant. BSS_FLUSH followed by SCAN command can
be used to clear all BSS entries from both the driver and
wpa_supplicant.

Signed-hostap: Jouni Malinen <j@w1.fi>