ring.cmd_sock is generic socket operation. Two most common uses are
setsockopt and getsockopt. This provides same interface as posix
versions of this methods.
libring has also [sqe_set_flags](https://man7.org/linux/man-pages/man3/io_uring_sqe_set_flags.3.html)
method. Adding that in our io_uring_sqe. Adding sqe.link_next method for setting most common flag.
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