io: use io_uring for all io

There should no longer be any

- read
- write
- connect
- sendmsg
- recvmsg
- accept

calls in the codebase. Previously we were using a mix of io_uring and
these calls which had some negative effects: Since we were using the old
system calls, we had to set the file descriptors to non-blocking. But
our io_uring code did not handle EAGAIN. This lead to programs sometimes
being killed when the wayland IO was actually blocking.

Now all file descriptors are set to blocking, but io_uring makes it
non-blocking from our perspective. The one exception are evdev files
because they are read via libinput and libinput uses the old system
calls.

src/compositor.rs

@@ -234,10 +234,10 @@ async fn start_compositor3(state: Rc<State>, test_future: Option<TestFuture>) {
 
     if backend.import_environment() {
         if let Some(acc) = state.acceptor.get() {
-            import_environment(&state, WAYLAND_DISPLAY, acc.socket_name());
+            import_environment(&state, WAYLAND_DISPLAY, acc.socket_name()).await;
         }
         for (key, val) in STATIC_VARS {
-            import_environment(&state, key, val);
+            import_environment(&state, key, val).await;
         }
     }