Libreswan: import version 5.1

Libreswan is an Internet Key Exchange (IKE) daemon for managing IPsec.

Libreswan supports IKEv1 and IKEv2 and has support for most of the
extensions (RFC + IETF drafts) related to IPsec, including IKEv2,
X.509 Digital Certificates, NAT Traversal, and many others.

On NetBSD and FreeBSD, Libreswan uses the PF_KEY_V2 IPsec stack.  On
Linux, Libreswan uses the XFRM IPsec stack.

Libreswan was forked from Openswan 2.6.38, which was forked from
FreeS/WAN 2.04. See the CREDITS files for contributor acknowledgments.