strongswan: new package

strongSwan is an OpenSource IPsec implementation for the Linux
operating system. It is based on the discontinued FreeS/WAN project
and the X.509 patch.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>