This brings io_uring helper methods to Zig for kernels >= 5.4.

We follow liburing's design decisions so that anyone who is comfortable with
liburing (https://unixism.net/loti/ref-liburing/index.html) will feel at home.

Thanks to @daurnimator for the first draft.

Refs: #3083
Signed-off-by: Joran Dirk Greef <joran@coil.com>

…lues

Now we're really future-proof... no more `opflags` creeping in.

When anonymous unions land, we can start using `accept_flags` etc.

Until then, code using this struct won't break when the kernel adds features.

Refs: #6349
Refs: #985

…) public

These will also be needed by any custom helpers

As per lib/libc/musl/arch/mips/bits/syscall.h.in:

```c
```

Ensures that the wakeup flag is read after the tail pointer has been
written. It's important to use memory load acquire semantics for the
flags read, otherwise the application and the kernel might not agree on
the consistency of the wakeup flag, leading to I/O starvation.

Refs: axboe/liburing@6768ddc
Refs: axboe/liburing#219

Decouples SQE queueing and SQE prepping methods to allow for non-sequential
SQE allocation schemes as suggested by @daurnimator.

Adds essential SQE prepping methods from liburing to reduce boilerplate.

Removes non-essential .link_with_next_sqe() and .use_registered_fd().

Removes non-essential .hardlink_with_next_sqe() and .drain_previous_sqes().

…FDCWD

If an older kernel fails the `openat` test because of `AT_FDCWD`
then we don't want to skip the `close` test.