gpython/pymain: -O, sys.executable, ...
- Teach gpython and pymain about
-; - Teach gpython and pymain about -O;
- Teach pymain to always set sys.executable, so that subprocess and multiprocessing work correctly out of the box;
- Fix ./bin/gpython crash;
- Rewrite options parsing via incremental getopt-like thing.
This changes are motivated by https://lab.nexedi.com/nexedi/slapos/merge_requests/862#note_121470 and https://lab.nexedi.com/nexedi/slapos/merge_requests/862#note_121600.