chmod: fix EROFS showing as Permission denied

[gabrielhnf]

Fixes #12196

Problem

When running chmod -R on a read-only filesystem, uutils was reporting:

chmod: cannot access 'file': Permission denied

Instead of the correct:

chmod: changing permissions of 'file': Read-only file system

Cause

safe_chmod_file was hardcoding ChmodError::PermissionDenied regardless of the actual error returned by the OS, discarding the real error kind.

Fix

Map ErrorKind::ReadOnlyFilesystem to a new ChmodError::ReadOnlyFileSystem variant with the correct message, consistent with GNU coreutils behavior.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)

[sylvestre]

looks good but could you please add a test to make sure we don't regress

[gabrielhnf]

Thanks for the suggestion! Also planning a follow-up PR to fix another problem regarding error reporting, should that include tests as well?