Add restart_ms option for automatic bus-off recovery

[CaptChrisD]

Summary

Adds a :restart_ms config option to VintageNetSocketCAN so CAN controllers can automatically recover from a bus-off condition.

When a CAN controller's transmit error counter crosses the bus-off threshold (e.g. it powers up before any other node is present to ACK its frames), it goes bus-off. With the kernel default restart-ms 0, it stays off until the interface is manually cycled — on a Nerves device that effectively means a reboot. Passing a positive restart-ms to ip link set ... type can lets the controller automatically rejoin the bus.

Changes

• Add :restart_ms (integer) to the validated options, defaulting to 0. The default preserves existing behavior (kernel default, auto-recovery disabled).
• Pass restart-ms <value> through to the ip link set ... type can up command.
• Replace the placeholder test with real coverage of normalize/1 and to_raw_config/3. Tests that shell out to ip are tagged :requires_ip and auto-excluded when ip isn't on PATH.
• Add a test-only config that redirects VintageNet's filesystem writes to a temp dir so mix test runs hermetically on a host.
• Document the supported options in the README.

Usage

config :vintage_net,
  config: [
    {"can0",
     %{
       type: VintageNetSocketCAN,
       vintage_net_socket_can: %{
         bitrate: 250_000,
         restart_ms: 100
       }
     }}
  ]