python: append paths from PYTHON<VERSION>PATH env var to sys.path

Every python interpreter that loads the recursive-pth-loader module will
now append paths to sys.path from PYTHON<VERSION>PATH environment
variable.

This means we can easily mix python2/3 environments and have modules
that are installed in a profile "activated" for the user by default. In
other words, python modules will be as easy to access from the python
shell as programs are to the (bash) shell.

For NixOS, the plan is to export PYTHON{26,27,32,33}PATH with
$PROFILE/lib/pythonVERSION/site-packages (for PROFILE in system_profile,
default_profile and user_profile, similarly as is done with PATH,
PERL5LIB and all other environment variables used to "activate" stuff
from profiles).