Kernel+LibC+UserspaceEmulator: Bring back sys$dup2()

This is racy in userspace and non-racy in kernelspace so let's keep
it in kernelspace.

The behavior change where CLOEXEC is preserved when dup2() is called
with (old_fd == new_fd) was good though, let's keep that.

DevTools/UserspaceEmulator/Emulator.h

@@ -138,6 +138,7 @@ private:
     ssize_t virt$getrandom(FlatPtr buffer, size_t buffer_size, unsigned int flags);
     int virt$sleep(unsigned);
     int virt$chdir(FlatPtr, size_t);
+    int virt$dup2(int, int);
     int virt$getpgrp();
     int virt$getpgid(pid_t);
     int virt$setpgid(pid_t pid, pid_t pgid);