re: #13327

Adds local interfaces to coder netcheck and checks their MTUs for potential problems.

This is mostly relevant for end-user systems where VPNs are common. We could also add it to coderd healthcheck, but until I see coderd connecting to workspaces over a VPN in the wild, I don't think its worth the UX effort.

Netcheck results get the following:

  "interfaces": {
    "error": null,
    "severity": "ok",
    "warnings": null,
    "dismissed": false,
    "interfaces": [
      {
        "name": "lo0",
        "mtu": 16384,
        "addresses": [
          "127.0.0.1/8",
          "::1/128",
          "fe80::1/64"
        ]
      },
      {
        "name": "en8",
        "mtu": 1500,
        "addresses": [
          "192.168.50.217/24",
          "fe80::c13:1a92:3fa5:dd7e/64"
        ]
      }
    ]
  }

Technically not back compatible if anyone is parsing coder netcheck output as JSON, since the original output is now under "derp" in the output.