HS 2.0: Add mechanism for disabling DGAF

HS 2.0: Add mechanism for disabling DGAF

disable_dgaf=1 in hostapd.conf can now be used to disable downstream
group-addressed forwarding (DGAF). In this configuration, a unique
GTK (and IGTK) is provided to each STA in the BSS to make sure the
keys do not match and no STA can forge group-addressed frames.

An additional mechanism in the AP needs to be provided to handle some
group-addressed frames, e.g., by converting DHCP packets to unicast
IEEE 802.11 frames regardless of their destination IP address and by
providing Proxy ARP functionality.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>