Add SADB_SAFLAGS_ESN flag

This flag is going to be used by IKE daemon to signal if
Extended Sequence Number feature is going to be used.

Value for this flag was taken from OpenBSD source code
https://github.com/openbsd/src/commit/6b4cbaf181c6b60701d9fb888fd0e7a4333eecbd

Submitted by: Patryk Duda <pdk@semihalf.com>
Reviewed by: ae
Differential revision: https://reviews.freebsd.org/D22366
Obtained from: Semihalf
Sponsored by: Stormshield