AP: Add support for Proxy ARP, DHCP snooping mechanism

AP: Add support for Proxy ARP, DHCP snooping mechanism

Proxy ARP allows the AP devices to keep track of the hardware address to
IP address mapping of the STA devices within the BSS. When a request for
such information is made (i.e., ARP request, Neighbor Solicitation), the
AP will respond on behalf of the STA device within the BSS. Such
requests could originate from a device within the BSS or also from the
bridge. In the process of the AP replying to the request (i.e., ARP
reply, Neighbor Advertisement), the AP will drop the original request
frame. The relevant STA will not even know that such information was
ever requested.

This feature is a requirement for Hotspot 2.0, and is defined in IEEE
Std 802.11-2012, 10.23.13. While the Proxy ARP support code mainly
resides in the kernel bridge code, in order to optimize the performance
and simplify kernel implementation, the DHCP snooping code was added to
the hostapd.

Signed-off-by: Kyeyoon Park <kyeyoonp@qca.qualcomm.com>