- Changed the xattr namespaces check to filter out system xattrs

- Changed the xattr namespaces check to filter out system xattrs
  instead of looking for specific namespace prefixes.  This will
  ensure that we send a namespace such as "security".
- Changed the positions of the xattr name & datum in the combined
  buffer so that we can change the name pointer without affecting
  the base pointer of the buffer (which needs to be unchanged so
  that we can free it).
- If a non-Linux system gets a non-user namespace, we now store
  namespace FOO as rsync.FOO.  This allows us to strip the rsync
  prefix when sending the values back.
- Fixed a memory leak when handling an error.
- Improved some of the variable names.