Validate at run-time that all required io_uring features are available, else disable the io_uring driver  #67

Support for io_uring was added in #24. The implementation detects, at run-time, whether the Linux kernel of the current machine supports io_uring, and if not, the driver is disabled. This run-time validation should be extended to probe io_uring to detects more of the kernel's capabilities and verify that each one (e.g. support for the "sendmsg" operation) is supported before the driver determines itself to be available.