hostapd: Add Multi-AP protocol support

hostapd: Add Multi-AP protocol support

The purpose of Multi-AP specification is to enable inter-operability
across Wi-Fi access points (APs) from different vendors.

This patch introduces one new configuration parameter 'multi_ap' to
enable Multi-AP functionality and to configure the BSS as a backhaul
and/or fronthaul BSS.

Advertise vendor specific Multi-AP capabilities in (Re)Association
Response frame, if Multi-AP functionality is enabled through the
configuration parameter.

A backhaul AP must support receiving both 3addr and 4addr frames from a
backhaul STA, so create a VLAN for it just like is done for WDS, i.e.,
by calling hostapd_set_wds_sta(). Since Multi-AP requires WPA2 (never
WEP), we can safely call hostapd_set_wds_encryption() as well and we can
reuse the entire WDS condition.

To parse the Multi-AP Extension subelement, we use get_ie(): even though
that function is meant for parsing IEs, it works for subelements.

Signed-off-by: Venkateswara Naralasetty <vnaralas@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>