Trigger soft lifetime expiration on sequence number

This patch adds 80% of UINT32_MAX limit on sequence number.
When sequence number reaches limit kernel sends SADB_EXPIRE message to
IKE daemon which is responsible to perform rekeying.

Submitted by: Patryk Duda <pdk@semihalf.com>
Reviewed by: ae
Differential revision: https://reviews.freebsd.org/D22370
Obtained from: Semihalf
Sponsored by: Stormshield