• Rename PYTHON_FEATURES to USE_PYTHON to comply to USE_PERL5 and to avoid a conflict in behaviour with the read-only COMPILER_FEATURES knob
• Fix the deprecated USE_PYTHON_BUILD and USE_PYTHON_RUN behaviour, which usually should be mutually exclusive, but some ports include both knobs

Phabric: D581
Recommended by: danfe@, makc@
Reviewed by: danfe, wg, antoine
Approved by: portmgr
With hat: python@