ARP: add feature arc

ARP: add feature arc

- arp-input, registered with the ethernet protocol dispatcher, performs
basic checks and starts the arc
- arp-reply; first feature on the arc replies to requests and learns
from responses (no functional change)
- arp-proxy; checks against the proxy DB

arp-reply and arp-proxy are enabled when the interface is appropriately
configured.

Change-Id: I7d1bbabdb8c8b8187cac75e663daa4a5a7ce382a
Signed-off-by: Neale Ranns <nranns@cisco.com>