Add ignore_old_scan_res configuration parameter

Add ignore_old_scan_res configuration parameter

This can be used to configure wpa_supplicant to ignore old scan results
from the driver cache in cases where such results were not updated after
the scan trigger from wpa_supplicant. This can be useful in some cases
where the driver may cache information for a significant time and the AP
configuration is changing. Many such cases are for testing scripts, but
this could potentially be useful for some WPS use cases, too.

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