rm: don't treat symlinks as write-protected

GNU rm does not check for write-protection on symbolic links; it
instead prompts to "remove symbolic link" regardless of the link's
permissions or its target's status.

This change:
- Ensures `prompt_file` checks for symlinks specifically using
  `symlink_metadata`, avoiding the incorrect "write-protected" prompt.
- Refactors permission checks into `is_writable_metadata` to allow
  using the already-fetched metadata, which also optimizes performance
  by reducing redundant `stat` calls.
- Updates `prompt_file_permission_readonly` to operate on metadata directly.

Author: GomesGoncalo

tests/by-util/test_rm.rs

@@ -1236,7 +1236,6 @@ fn no_preserve_root_may_not_be_abbreviated() {
 
     assert!(at.file_exists(file));
 }
-
 #[cfg(unix)]
 #[test]
 fn test_symlink_to_readonly_no_prompt() {