jls: IPv4/v6 should use a hyphen with libxo

[jlduran]

As stated in libxo's best practices, field names should be kebab-cased, it is better for XML output and JSON output can always use the XOF_UNDERSCORES flag to generate underscores.

Before:

% jls --libxo json,pretty
{
  "__version": "2",
  "jail-information": {
    "jail": [
      {
        ...
        "ipv4_addrs": [],
        "ipv6_addrs": []
      }
    ]
  }
}

After:

% jls --libxo json,pretty
{
  "__version": "3",
  "jail-information": {
    "jail": [
      {
        ...
        "ipv4-addrs": [],
        "ipv6-addrs": []
      }
    ]
  }
}

Or, if underscores are preferred:

% jls --libxo json,underscore,pretty
{
  "__version": "3",
  "jail_information": {
    "jail": [
      {
        ...
        "ipv4_addrs": [],
        "ipv6_addrs": []
      }
    ]
  }
}

[jlduran]

Note this change warrants a bump in the API version.

More tests are required before submitting it upstream.