redox-drm: graceful exit when scheme:drm already registered

When pcid-spawner launches redox-drm for a GPU device, the init
service (10_redox-drm.service) may also start a second instance.
The guard check in the service file (head -c 1 /scheme/drm/card0)
has a race condition with pcid-spawner's scheme registration.

Move the scheme:drm existence check into the binary itself. If
scheme:drm is already registered when run() starts, log and exit
gracefully with daemon.ready() instead of crashing with a fatal
"no GPU found" error.

local/recipes/gpu/redox-drm/source/src/main.rs

@@ -59,6 +59,12 @@ fn init_logging(level: LevelFilter) {
 }
 
 fn run(daemon: daemon::Daemon) -> Result<()> {
+    if is_scheme_registered("drm") {
+        info!("redox-drm: scheme:drm already registered, another instance is active");
+        daemon.ready();
+        return Ok(());
+    }
+
     let info = select_gpu_from_args()?;
     verify_supported_gpu(&info)?;
 
@@ -174,6 +180,10 @@ fn create_drm_socket() -> Result<Socket> {
         .map_err(|e| DriverError::Initialization(format!("failed to register drm scheme: {e}")))
 }
 
+fn is_scheme_registered(name: &str) -> bool {
+    std::fs::read_dir(format!("/scheme/{}", name)).is_ok()
+}
+
 fn select_gpu_from_args() -> Result<PciDeviceInfo> {
     let mut args = env::args().skip(1);
     let parsed = match (args.next(), args.next(), args.next()) {