Feature: `io_uring_register` with fd=-1

[oxalica]

There are some io_uring_register opcodes which do not operate on an existing ring and require the first argument FD to be -1, eg. IORING_REGISTER_SEND_MSG_RING, IORING_REGISTER_QUERY and more.

All these "blind registration" opcodes can be seen in kernel source: https://github.com/torvalds/linux/blob/f4d0ec0aa20d49f09dc01d82894ce80d72de0560/io_uring/register.c#L981-L999
If fd is not -1, the syscall will immediately fail with EINVAL. Our current API takes an impl AsFd which guarantees the validity of fd, since BorrowedFd forbids itself to be -1. This means IoringRegisterOp::RegisterSendMsgRing is actually unusable with our current APIs.