Adding initial support for IKEv2 Site-to-Site VPNs

This patch adds support for IKEv2 based Site-to-Site IPSec VPNs.

This is achieved by forcing StrongSwan's charon daemon which handles IKEv2 negotiations to start. To prevent unexpected behavior from the charon IKEv2 daemon, I've added code to automatically force IKEv1 negotiations instead of having StrongSwan decide on using IKEv1 or IKEv2 when a peer sends in a proposal. However, if the user chooses, they can have StrongSwan decide to automatically use either IKEv1 or IKEv2.