Issue1493

As a root user, when calling tarfile.extractall() to extract a tarball
containing files owned by non-root, tarfile module throws the following
exception:

  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1531, in extractall
    self.extract(tarinfo, path)
  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1568, in extract
    self._extract_member(tarinfo, os.path.join(path, tarinfo.name))
  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1568, in extract
    self._extract_member(tarinfo, os.path.join(path, tarinfo.name))
  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1660, in
_extract_member
    self.chown(tarinfo, targetpath)
  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1772, in chown
    os.chown(targetpath, u, g)
  File "/home/tzhou/jython2.5.0/Lib/tarfile.py", line 1772, in chown
    os.chown(targetpath, u, g)

I looked at tarfile.py. It appears that if the euid of the calling user
is 0 (root), chown() method (which is called by extractall()) will
invoke os.chown(). The latter is not implemented by Jython. 

FYI, I reproduced this on both a Ubuntu 8 server and Redhat EL 5.2
server. Jython interpreter version:

Jython 2.5.0 (Release_2_5_0:6476, Jun 16 2009, 13:33:26) 
[Java HotSpot(TM) Server VM (Sun Microsystems Inc.)] on java1.6.0_07

I've added os.chown in r6920 which our posix lib already supports, it even 
falls back to forking a chown process when not using the native bits. So 
we don't need to hasattr check it in tarfile