Rock Ridge writing support

[Kamil Domański]

• 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