Revert the (default) ip_id algorithm to the pre-randomid algorithm,
due to demonstrated low-period repeated IDs from the randomized IP_id
code. Consensus is that the low-period repetition (much less than
2^15) is not suitable for general-purpose use.
Allocators of new IPv4 IDs should now call the function ip_newid().
Randomized IP_ids is now a config-time option, "options RANDOM_IP_ID".
ip_newid() can use ip_random-id()_IP_ID if and only if configured
with RANDOM_IP_ID. A sysctl knob should be provided.
This API may be reworked in the near future to support linear ip_id
counters per (src,dst) IP-address pair.
de80d1419e