pefs status report 8

[Gleb Kurtsou]

Weekly progress:
- Support for changing directory key. pefs setkey /path/dir. At lower
  level it renames directory, reencrypting directory name with new key.
  Directory contents left unchanged.
- Implement key chaining. Key chains are stored in db file .pefs at
  filesystem root. key id (key fingerprint) is used as db key and data
  stored is next key in chain. Due to such design any key can only be part
  of single chain.
- Fixed vop_link. Creation of link to unencrypted files is now
  forbidden. Link is created with the same key and tweak as source file.