relibc: fix mutex_owner_id_is_live Sys::open/close return type

Sys::open returns Result<i32, Errno> (not i32), and Sys::close
returns Result<i32, Errno> as well. The previous version of
mutex_owner_id_is_live used the cstr.as_ptr() pattern (which
the cross-compile correctly rejected because Sys::open expects
NulStr<'_, Thin> not *const c_char) and treated the Result as
a raw fd (which the type system rejected).

Fix: pass the CStr directly (it converts to NulStr via Deref)
and match on the Result<>'s Ok/Err variants instead of doing
fd >= 0 comparison on a non-i32 type.

src/pthread/mod.rs

@@ -481,17 +481,18 @@ pub fn mutex_owner_id_is_live(owner: u32) -> bool {
         // For Redox, attempt to open the thread's "name" handle via
         // the proc scheme. If the thread is alive, the open succeeds;
         // if it has exited and been reaped, the open returns ENOENT.
-        let path = format!("proc:{}/name", owner);
+        let path = alloc::format!("proc:{}/name\0", owner);
         let cstr = match crate::c_str::CStr::from_bytes_with_nul(path.as_bytes()) {
             Ok(c) => c,
             Err(_) => return false,
         };
-        let fd = crate::platform::Sys::open(cstr.as_ptr(), crate::header::fcntl::O_RDONLY, 0);
-        if fd >= 0 {
-            let _ = crate::platform::Sys::close(fd);
-            true
-        } else {
-            false
+        let fd = crate::platform::Sys::open(cstr, crate::header::fcntl::O_RDONLY, 0);
+        match fd {
+            Ok(fd) => {
+                let _ = crate::platform::Sys::close(fd);
+                true
+            }
+            Err(_) => false,
         }
     }
     #[cfg(target_os = "linux")]