input/uring: new input plugin using io_uring

This is the final piece of the series to establish io_uring support on
Linux.

MPD doesn't need io_uring for its efficient bulk I/O support, but to
allow file I/O to be cancelled.  This is a big problem on CIFS/NFS
mounts where processes sleep uninterruptable if the file server
disappears, deadlocking MPD.

With io_uring, a flaky NFS connection allows MPD to continue to work
(even though there are still deadlocks inside MPD which need to be
addressed).

This plugin does not yet use cancellable `open()` using
`IORING_OP_OPENAT`.  This will be implemented later.

Lots of other optimization opportunities for io_uring are still
missing as well - for example the database update could benefit a lot,
but unfortunately, io_uring doesn't have `readdir()` support just yet.

src/input/Init.cxx

@@ -30,9 +30,18 @@
 #include <cassert>
 #include <stdexcept>
 
+#include "io/uring/Features.h"
+#ifdef HAVE_URING
+#include "plugins/UringInputPlugin.hxx"
+#endif
+
 void
 input_stream_global_init(const ConfigData &config, EventLoop &event_loop)
 {
+#ifdef HAVE_URING
+	InitUringInputPlugin(event_loop);
+#endif
+
 	const ConfigBlock empty;
 
 	for (unsigned i = 0; input_plugins[i] != nullptr; ++i) {