ktls: Capture initial tls seqno at time offload is initiated

Some drivers want the TLS seqno when offload starts. Capture this
for them by adding a union for initial_seqno, sharing space with
the TLS 1.0 next_seqno.

Reviewed by: jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D54275
MFC After: 1 month

(cherry picked from commit 4d692068f6850282dabde9e35b3098a4ebec9592)