Merge pull request #15 from oniko/xattrs

- clarify note about xattribute namespaces in xattrs2.cc test
- implemented secure versions of listxattr and getxattr in SDir
- protect CWD in SDir by mutex
- use enum type instead of int
- use a define for config and subvolume
- make it even less work to have two testsuites, e.g. for btrfs and lvm
- handle change (from 0 to >0) of v_size between getxattr calls
- added testcase for xattrs with empty value
- override unneeded passwd entry
- minor updates
- fix typo in error message