AP: Allow identifying which passphrase station used with wpa_psk_file

AP: Allow identifying which passphrase station used with wpa_psk_file

It is now possible to optionally specify keyid for
each wpa_psk_file entry:

 keyid=something 00:00:00:00:00:00 secretpassphrase

When station connects and the passphrase it used
has an associated keyid it will be appended to the
AP-STA-CONNECTED event string:

 wlan0: AP-STA-CONNECTED 00:36:76:21:dc:7b keyid=something

It's also possible to retrieve it through the control interface:

 $ hostapd_cli all_sta
 Selected interface 'ap0'
 00:36:76:21:dc:7b
 ...
 keyid=something

New hostapd is able to read old wpa_psk_file. However, old hostapd will
not be able to read the new wpa_psk_file if it includes keyids.

Signed-off-by: Michal Kazior <michal@plume.com>