o Use new function ip_fillid() in all places throughout the kernel,

where we want to create a new IP datagram.

o Add support for RFC6864, which allows to set IP ID for atomic IP

datagrams to any value, to improve performance. The behaviour is
controlled by net.inet.ip.rfc6864 sysctl knob, which is enabled by
default.

o In case if we generate IP ID, use counter(9) to improve performance.
o Gather all code related to IP ID into ip_id.c.

Differential Revision: https://reviews.freebsd.org/D2177
Reviewed by: adrian, cy, rpaulo
Tested by: Emeric POUPON <emeric.poupon stormshield.eu>
Sponsored by: Netflix
Sponsored by: Nginx, Inc.
Relnotes: yes