Add non-blocking read mode to SocketCan CanRaw

[Copilot]

CanRaw.TryReadFrame always blocks until a frame arrives, with no way to poll the bus. This adds an opt-in non-blocking mode (the O_NONBLOCK / MSG_DONTWAIT equivalent).

Changes

• CanRaw.Blocking property — defaults to true (existing behavior). Set to false to switch the socket to non-blocking mode via fcntl(O_NONBLOCK).
• Interop.Read — now uses SetLastError and treats EAGAIN/EWOULDBLOCK as "no data" (returns a sentinel) instead of throwing.
• TryReadFrame — returns false immediately when no frame is available in non-blocking mode; the timestamp overload skips the SIOCGSTAMP ioctl when no frame was read.
• README — added a non-blocking usage example.

Usage

using CanRaw can = new CanRaw();
can.Blocking = false;
byte[] buffer = new byte[8];

while (true)
{
    if (can.TryReadFrame(buffer, out int frameLength, out CanId id))
    {
        // process frame
    }
    else
    {
        // no frame available right now — do other work instead of blocking
        Thread.Sleep(10);
    }
}

Notes

• Default callers are unaffected; non-blocking is strictly opt-in.
• In non-blocking mode false now also signals "no frame available" in addition to the existing "invalid frame" case — both indicate no usable frame was returned.
• The fcntl flag constants are Linux-specific, consistent with this Linux-only device binding.

[raffaeler]

It would be better to test this on a real project.
Any volunteer?

[krwq]

@copilot please rename Blocking property it so it has Read and something with waiting in the name. Do research what good descriptive name could be. Also change interop method name to match the new name

[Copilot]

Renamed Blocking to WaitForFrameOnRead (includes Read and the waiting concept) and renamed the interop method to SetWaitForFrameOnRead to match. Done in c6702b5.