Site-to-site VPN support for NAT-T

Site-to-site IKE gateway configuration for Network Address Translation-Traversal (NAT-T) is now supported on the server side (IKE responder). This is in addition to the current implementation of NAT-T support for dynamic IKE gateway configuration. A remote-identity value is used to validate a peer's ike-id or id during Phase 1 of IKE tunnel negotiation.