Add support for multi-SSID scan requests

Add support for multi-SSID scan requests

If the driver reports support for more than one SSID per scan request,
optimize scan_ssid=1 operations in ap_scan=1 mode. This speeds up
scanning whenever scan_ssid=1 is used since the broadcast SSID can be
included in every scan request and if driver supports more than two
SSIDs in the scan request, the benefits are even larger when multiple
networks have been configured with ap_scan=1.

This is also cleaning up wpa_supplicant_scan() function by moving code
around so that the SSID list is not processed unnecessarily if the
operation mode does not need this.