Keep list of lagg ports sorted by if_index.

Obtained from:	Yandex LLC
MFC after:	1 week
Sponsored by:	Yandex LLC
drop unnecessary ifdef IPSEC's.  This file is only compiled when IPSEC
is defined...

Differential Revision:	D839
Reviewed by:	bz, glebius, gnn
Sponsered by:	EuroBSDCon DevSummit
Make all lagg protocol methods live in lagg_protos, not in softc. All
interfaces of a same protocol, use the same methods.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.