File cps/main.c is the only user of functions kni_process_arp() and
kni_process_nd(). Moreover, the only relationship between
these functions and KNI is the fact that they process packets
read from KNI interfaces.

This commit
1. moves kni_process_arp() and kni_process_nd() from
   cps/kni.c to cps/main.c;
2. makes those functions static; and
3. renames those functions to process_arp() and process_nd().