Add io_uring_register flags

[dmitry-salin]

Adds IoringRegisterFlags to rustix::io_uring::io_uring_register.

int io_uring_register(unsigned int fd, unsigned int opcode, void *arg, unsigned int nr_args);

fd is the file descriptor returned by a call to io_uring_setup. If opcode has the flag IORING_REGISTER_USE_REGISTERED_RING ored into it, fd is instead the index of a registered ring fd.

Resources: liburing, kernel