fake_id0: improve (r|e|s)(u|g)id support

The extension is now tracking (real, effective and saved) user and group ids.

This commit allows to run samba4 configure succefully:

Before
  $ proot -w samba-4.1.0 -0 /bin/sh configure
  [...]
  Checking whether Linux should use 32-bit credential calls                                       : not found
  Checking whether we can use Linux thread-specific credentials                                   : not found
  [...]
  Checking whether setuidx is available : samba-4.1.0/source3/wscript:997: error: the configuration failed (see 'samba-4.1.0/bin/config.log')

After
  $ proot -w samba-4.1.0 -0 /bin/sh configure
  [...]
  Checking whether Linux should use 32-bit credential calls                                       : not found
  Checking whether we can use Linux thread-specific credentials                                   : ok
  [...]
  'configure' finished successfully
Merge pull request #33 from ivoire/improve_fake_id0

fake_id0: improve (r|e|s)(u|g)id support