Open special files in another thread

[ysbaddaden]

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We can't detect without significant overhead whether the file at path might block for a while before calling open.

While O_NONBLOCK has no effect on regular disk files, special file types are a different story. Open with O_NONBLOCK will fail with ENXIO for O_WRONLY (no connected reader) but it will always succeed for O_RDONLY (regardless of a connected writer or not), then any attempt to read will return EOF, leaving no means to wait until a writer connects.

I thus rely on the blocking arg: when nil the file might be a special file type, so I check it's type, when false I consider it's a special file. If the file is a fifo (named pipe) or a character device, I open it in another thread to not risk blocking the current thread (and thus other fibers) until a reader or writer is also connected.

Opening regular files is unaffected: we still open directly, while opening a special file will now avoid blocking the current thread.

NOTE: we need the preview_mt flag to safely re-enqueue the current fiber from the bare thread.

Depends on #15754 and #15767.