MFC r327295:

MFC r327295:
Start syncing changes from OpenBSD's ip6_id.c instead of ip_id.c.

correct non-repetitive ID code, based on comments from niels provos.
- seed2 is necessary, but use it as "seed2 + x" not "seed2 ^ x".
- skipping number is not needed, so disable it for 16bit generator (makes
  the repetition period to 30000)

Obtained from:	OpenBSD (CVS rev. 1.2)

git-svn-id: svn://svn.freebsd.org/base/stable/10@327550 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f