Fix handling of PYTHON_USE_WITH with more than 2 USE flags.

Fix handling of PYTHON_USE_WITH with more than 2 USE flags.
Improve python_pkg_setup().
Rename python_clean_sitedirs() to python_clean_installation_image().
Improve python_clean_installation_image().
Improve handling of -- option in phase functions.
Add check for dev-lang/python in validate_PYTHON_ABIS() (bug #319651).
Improve error message in python_copy_sources().
Improve python_set_active_version().
Improve error messages in PYTHON().
Support PEP 3147 in python_mod_optimize(), python_mod_cleanup() and python_mod_compile().