security/strongswan: add vici-based configuration for the rc script

The rc script is modified to allow both a legacy (ipsec.conf-based)
startup or a new (swanctl.conf-based) config. Default is the legacy.

The new setup is based on vici, the Versatile IKE Configuration Interface.

For more details, see:

https://wiki.strongswan.org/projects/strongswan/wiki/Vici

PR: 234648
Submitted by: Jose Luis Duran <jlduran@gmail.com>
Reviewed by: Sam Chen <sc.gear@one.caeon.com>
Approved by: strongswan@Nanoteq.com (maintainer)
Differential Revision: D19367