wpa_supplicant: Support Multi-AP backhaul STA onboarding with WPS

wpa_supplicant: Support Multi-AP backhaul STA onboarding with WPS

The Wi-Fi Alliance Multi-AP Specification v1.0 allows onboarding of a
backhaul STA through WPS. To enable this, the backhaul STA needs to add
a Multi-AP IE to the WFA vendor extension element in the WSC M1 message
that indicates it supports the Multi-AP backhaul STA role. The Registrar
(if it support Multi-AP onboarding) will respond to that with a WSC M8
message that also contains the Multi-AP IE, and that contains the
credentials for the backhaul SSID (which may be different from the SSID
on which WPS is performed).

Introduce a new parameter to wpas_wps_start_pbc() and allow it to be
set via control interface's new multi_ap=1 parameter of WPS_PBC call.
multi_ap_backhaul_sta is set to 1 in the automatically created SSID.
Thus, if the AP does not support Multi-AP, association will fail and
WPS will be terminated.

Only wps_pbc is supported.

This commit adds the multi_ap argument only to the control socket
interface, not to the D-Bus interface.

Since WPS associates with the fronthaul BSS instead of the backhaul BSS,
we should not drop association if the AP announces fronthaul-only BSS.
Still, we should only do that in the specific case of WPS. Therefore,
add a check to multi_ap_process_assoc_resp() to allow association with a
fronthaul-only BSS if and only if key_mgmt contains WPS.

Signed-off-by: Davina Lu <ylu@quantenna.com>
Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Cc: Marianna Carrera <marianna.carrera.so@quantenna.com>