/etc/profile: export PYTHON<VERSION>PATH env vars

Export PYTHON{26,27,32,33}PATH environment variables, similarly to how
we do it with PATH, PERL5LIB etc..

This enables Python interpreters to automatically pick up modules
installed in the system path, the default profile and the user profile.

Will this ruin purity?
- This will not affect nixpkgs builders because they don't source
  /etc/profile (they are pure).
- It will "extend" the environment of wrapped python programs (unless
  they unset PYTHON<VERSION>PATH), so that potential
  runtime-dependencies may be uncaught. But this is no worse than what
  happens with PATH and PERL5LIB environment variables today.