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/backends/metal.rs

@@ -211,7 +211,7 @@ fn dup_fd(fd: c::c_int) -> Result<Rc<OwnedFd>, MetalError> {
 }
 
 pub async fn create(state: &Rc<State>) -> Result<Rc<MetalBackend>, MetalError> {
-    let socket = match state.dbus.system() {
+    let socket = match state.dbus.system().await {
         Ok(s) => s,
         Err(e) => return Err(MetalError::DbusSystemSocket(e)),
     };