fix(spawn): leak child thread fd to prevent race with dynamic linker

posix_spawn creates a new context and returns its thread fd to the
parent. The kernel currently does not keep a self-reference to the child
context's thread fd, so when the parent closes its handle, the child's
thread object may be destroyed before the dynamic linker reads
AT_REDOX_THR_FD from auxv and opens regs/env. Leak the parent handle
until the kernel is fixed to retain a reference per context.

src/platform/redox/mod.rs

@@ -1483,6 +1483,13 @@ impl Pal for Sys {
         let start_fd = child.thr_fd.dup_into_upper(b"start")?;
         start_fd.write(&[0])?;
 
+        // The kernel does not keep a self-reference to the new context's thread
+        // fd. If the parent closes its handle before the child reads AT_REDOX_THR_FD
+        // from auxv and creates its own reference, the child thread object is
+        // destroyed and the dynamic linker panics trying to open regs/env. Until
+        // the kernel holds a reference per context, leak the parent handle.
+        let _ = child.thr_fd.take();
+
         Ok(pid_t::try_from(child.pid).unwrap())
     }