Rock Ridge writing support

[kdomanski]

• [ ] make RR writing the default, but allow disabling it

  The RRIP defines the following System Use Entries:

• [ ] PX POSIX file attributes
• [ ] PN POSIX device number
• [ ] SL Symbolic link
• [ ] NM Alternate name
• [ ] CL Child link
• [ ] PL Parent link
• [ ] RE Relocated directory
• [ ] TF Time stamp(s) for a file
• [ ] SF File data in sparse file format