add support for L2TP client-auth-id

[GIC-de]

Add support to select L2TP server by client-auth-id (SCCRQ hostname). This allows to setup multiple tunnels on the same IP address.

Example with new client-auth-id:

{
  "l2tp-server": [
    {
      "name": "LNS1S",
      "client-auth-id": "L1S",
      "address": "100.0.0.11",
      "secret": "test1s",
      "receive-window-size": 64,
      "data-control-priority": true,
      "data-control-tos": 224
    },
    {
      "name": "LNS2S",
      "client-auth-id": "L1S",
      "address": "100.0.0.12",
      "secret": "test2s",
      "receive-window-size": 64,
      "data-control-priority": true,
      "data-control-tos": 224
    },
    {
      "name": "LNS1",
      "address": "100.0.0.11",
      "secret": "test1",
      "receive-window-size": 64,
      "data-length": false,
      "data-offset": false,
      "data-control-priority": true,
      "data-control-tos": 224
    },
    {
      "name": "LNS2",
      "address": "100.0.0.12",
      "secret": "test2",
      "receive-window-size": 64,
      "data-length": true,
      "data-offset": false,
      "data-control-priority": true,
      "data-control-tos": 224
    }
  ]
}