New driver capability info: max number of scan SSIDs

New driver capability info: max number of scan SSIDs

The driver wrappers can now inform wpa_supplicant how many SSIDs can
be used in a single scan request (i.e., send multiple Probe Requests
per channel). This value is not yet used, but it can eventually be used
to allow a new scan command to specify multiple SSIDs to speed up
scan_ssid=1 operations. In addition, a warning could be printed if
scan_ssid=1 is used with a driver that does not support it
(max_scan_ssids=0).