Update IKE

[allen0091]

• Add decrypt and encrypt
• Add sha256 algorithm
• Remove XFRM related code

[ianchen0119]

Please resolve the conflicts.

[tim-ywliu]

return (*IKEHeader, error)

[tim-ywliu]

ikeHdr *IKEHeader

[tim-ywliu]

ikeMsg := new(message.IKEMessage)
if ikeHdr == nil {
    err := ikeMsg.Decode(msg)
} else {
    ikeMsg.IKEHeader = ikeHdr
    err := ikeMsg.DecodePayload(msg[msg[message.IKE_HEADER_LEN:])
}