re:#13327

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

This is mostly relevant for end-user systems where VPNs are common. Wecould 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 parsingcoder netcheck output as JSON, since the original output is now under"derp" in the output.