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))
gentoo-python: Remove Python 2

Portage runs fine on Python 3, and there's no need to have multiple
versions of Python installed.  Removing it here allows us to trim down
to smaller images, because Python's pretty heavy:

  $ docker run -i -t --rm wking/gentoo-portage:20140717 /bin/bash
  d1cdbe4c737a / # du -hs /usr/lib64/python2.7/
  66M     /usr/lib64/python2.7/

My current trimmed ngircd is only image is 341 MB:

  $ docker images wking/ngircd-trimmed | grep 20140717
  wking/ngircd-trimmed   20140717   98daec1a09c5   16 hours ago   340.9 MB

so about 20% of the trimmed image is a useless old Python version.  We
could shrink the trimmed image even more by removing Portage and
Python 3, but that requires some external-to-Portage hackery.  This is
easy to do, and folks who *do* need Python 2 can easily adjust their
PYTHON_TARGETS, PYTHON_SINGLE_TARGET, eselected Python, and re-emerge.

We only need to emerge @system, because @world is still empty at this
point.