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)
MFC after: 1 week