trim.sh: Add virtual/package-manager to @world

trim.sh: Add virtual/package-manager to @world

We don't really need this, but it's easier to avoid trouble if we're
not using the package manager to uninstall itself and it's
dependencies ;).  Portage will protect itself and refuse to unmerge
itself and the version of Python it's using, but it doesn't protect
other dependencies like app-admin/eselect-python (which supplies the
/usr/bin/python-wrapper linked from /usr/bin/python).

I've also added sys-apps/findutils to avoid crashing with:

  Traceback (most recent call last):
    File "/usr/bin/emerge", line 50, in <module>
      retval = emerge_main()
    File "/usr/lib64/portage/pym/_emerge/main.py", line 1031, in emerge_main
      return run_action(emerge_config)
    File "/usr/lib64/portage/pym/_emerge/actions.py", line 3983, in run_action
      emerge_config.trees, emerge_config.target_config.mtimedb, rval)
    File "/usr/lib64/portage/pym/portage/proxy/objectproxy.py", line 31, in __call__
      return result(*args, **kwargs)
    File "/usr/lib64/portage/pym/_emerge/post_emerge.py", line 146, in post_emerge
      chk_updated_cfg_files(settings['EROOT'], config_protect)
    File "/usr/lib64/portage/pym/_emerge/chk_updated_cfg_files.py", line 17, in chk_updated_cfg_files
      portage.util.find_updated_config_files(target_root, config_protect))
    File "/usr/lib64/portage/pym/portage/util/__init__.py", line 1735, in find_updated_config_files
      stderr=subprocess.STDOUT)
    File "/usr/lib64/python2.7/subprocess.py", line 709, in __init__
      errread, errwrite)
    File "/usr/lib64/python2.7/subprocess.py", line 1326, in _execute_child
      raise child_exception
  OSError: [Errno 2] No such file or directory

the trouble comes from portage.util.find_updated_config_files, which
is calling find with code like:

  if stat.S_ISDIR(mymode):
      mycommand = \
          "find '%s' -name '.*' -type d -prune -o -name '._cfg????_*'" % x
  else:
      mycommand = "find '%s' -maxdepth 1 -name '._cfg????_%s'" % \
                  os.path.split(x.rstrip(os.path.sep))