Bugfix: Fix problems in f_file exposed by unit tests.

Bugfix: Fix problems in f_file exposed by unit tests.

Add _f_file_rename_use_renameat2_ to enable support for renameat2() for systems in which this is available.
Add flags parameter to rename function to automatically support this.

Add missing "!path.used" checks.
Add missing parameter checks.
Move parameter checks that should instead be file closed checks that process normally.
Consistently apply file closed checks.

The function private_f_file_role_change_at() needs to check that result is not an error before processing group.
Failure to do this may result in hiding an error.

Minor clean ups.